Here are the reasons why I don't believe in UFOs:
(1) While one cannot say what aliens will look like specifically one can say what they will not look like. In particular, they will not look like humans. While the biological processes that produce life are the same, the evolutionary forces are not. Therefore, while one can expect a biological organism composed of cells based on a hydrocarbon protein structure it is not reasonable to expect that the end result of millions of years of evolution in two separate environments would produce sentient beings that closely resemble each other. Chimpanzees are an example of the improbability of such an event even here on Earth. They have 99% of their DNA identical to us but even that 1% difference results in an appearance so dramatically different as to be obviously non-human. Imagine the difference that a completely separate evolutionary process would create!
Furthermore, the gravitational field of the alien's world would have to be similar to that of Earth to produce beings of a scale similar to our own and a cellular structure leading to similar eyes, hands, bipedal motion, organ configuration and so on. The evolution of such a being would require billions of years in a stable, oceanic environment approximately the size of Earth orbiting a stable sun at a distance that would produce a climate similar to ours allowing liquid water to exist, an atmosphere which is oxygen rich allowing chemical reactions to occur, a mineral content which is non-toxic, etc. The probability that such a planet exists within even 1000 light years of Earth is extremely small considering that no radio signals have been found within the last 20 years of searching and most nearby (<100 lyr) stars have been found to be devoid of such planets.
(2) The distance travelled from one inhabited star to another is too great to be performed by living beings. It requires tremendous time and fuel supplies, a sustainable food source, a sufficient population to survive across generations, repair materials and so on. If we examine these one by one, the difference between observed UFOs and what simple Physics dictates is apparent. Firstly, there are no Earth-like planets within 100 light-years of us. In fact, there are very few stars within 100 light-years of Earth. This means that any aliens would have to come from much greater distances. To travel even a few light years would take several years at speeds near the speed of light (say 30%-50%). In order to achieve these speeds, as well as provide an artificial gravity, a ship would have to accelerate at 1 G for half the trip then decelerate for the next half. This pre-supposes that the aliens know exactly where they are going and travel directly here without interruption that, in itself, is not a particularly probable circumstance but more importantly it requires an enormous amount of fuel. The only fuel capable of providing such power as would be needed to perform this task would be anti-matter. For a 100 light year trip this would amount to about 50 lbs of anti-matter - presuming they would wish to return home. This might not seem like a great deal of material except for the small detail that there isn't any free anti-matter available in the Universe. It must all be manufactured. Currently, the largest cyclotron on Earth has the capability to produce about a millionth of a gram a year but let's pretend that by some technological miracle anti-matter can be produced easily.
The next problem is to contain it; that can be done only magnetically. For 50 lbs of anti-matter, an iron magnet the size of several large office buildings would be required. If, again, by some miracle of technology the problem of building a powerful enough magnet could be overcome the magnetic field it produced would still be so intense as to require being heavily shielded and kept far away from the mother ship to avoid affecting other instrumentation or biological life. This, of course, does not fit the descriptions provided of alien craft.
(3) The accelerations reported for some UFOs is so great as to prevent sentient life from existing. The maximum acceleration in which any life form that evolved in a gravitational field approximating the strength of Earth's can survive is about 10 G's. This would mean a 100 lb alien would weigh 1/2 ton! Any living being that was subjected to repeated accelerations of 10 G's, even for short periods, would die of internal injuries quite quickly even if they could remain conscious and in control of their craft. The craft itself would not be able to withstand much such punishment as metal fatigue would quickly become a problem. It's also difficult to see the necessity of aliens subjecting themselves to such torture unless it was a matter of life and death yet these accelerations are routinely reported.
(4) The normal saucer-shaped depictions of UFOs, together with the cigar shaped ones are aerodynamically unstable and unsuitable for flight in an atmosphere without fins of some sort. These shapes are also unnecessary in the vacuum of space. No intelligent beings with any knowledge of geometry and aerodynamics would design such a shape for a craft intended for use in an atmosphere. Furthermore, in light of the enormous requirements for fuel, food and population that an interstellar ship would have the size is likely to be on the order of a small town. Such a craft would not be able to evade visual detection for long, if at all. This size of craft would not be able to fly in the Earth's atmosphere and is inconsistent with reports of UFO size. Therefore, this mother ship would also have to carry shuttle craft with its own power systems. Since they are so much smaller they would need to be shaped for atmospheric flying even with anti-matter powered engines otherwise they would simply burn up or blow apart due to friction and turbulence.
(5) The human mind is capable of fooling itself very easily because perception is distorted by our experience. When a human encounters a phenomenon that is outside his experience his mind will attempt to understand it in terms of known ideas. This is the basis for optical illusions, aliens, gods, ghosts or other supernatural concepts. Of course, past civilizations have believed in many gods that control natural phenomena - at least until a scientific explanation was found. Belief in the supernatural is a way of shifting responsibility away from oneself. If something happens, well, then it's God's will - not my fault. Psychics are a means for people to make decisions without actually bearing the responsibility for the results. These "beliefs" are part of the human psyche - a way of processing information.
Identification of unusual natural phenomena as UFOs is a problem of perception. If one is willing to believe raw numbers then 10,000 UFO reports should be a good indication that there really is something there. Unfortunately, when project Blue Book investigated these reports it could explain 95% as easily identifiable natural phenomena. This overwhelming rate of identification is a very strong indication that what is really happening is that the UFO reports are unreliable because people are placing an interpretation on events which is the result of ignorance, wishful thinking, gullibility or other unreliable methods. It is well-known by Police that five witnesses to an incident will give five different versions of what happened - none of which will be entirely accurate.
This principle also applies to photographs and tapes of such events that are always of distant, unclear objects. There is not a single clear photograph in existence of anything that can reliably be said to be an alien spacecraft. Furthermore, there is no debris, garbage, footprints, metal fragments or ground markings that have ever been found and identified as alien in origin. The high proportion of hoaxes, whether perpetrated by others or by the mind on itself, is a strong indicator that there is a big difference between reality and perception. If one is willing to accept virtually anything as "evidence" then virtually anything becomes plausible.
It is also worth noting that sightings of actual aliens didn't begin until after the movies and comic books started portraying them. In the beginning most descriptions of aliens varied widely but now that we've had a few decades to get used to the idea they are becoming more consistent with the original movie versions of the 1950's. It's long been well-known that adults who recall past lives are just recalling previous things they've read or seen as a child and the same phenomena is at work with aliens as well. Mass hallucinations are also well known. There is a case in India where 10,000 people swore they could see the Sun dance across the sky. Unfortunately, the other 5 billion of us didn't see that. There has also been a film taken of a roomful of people who believe an inanimate object moved. The film clearly showed the object at rest and the people moving. So much for perception or experience being reliable.
(6) Crop circles are another example of a hoax used to indicate the presence of "aliens". Unfortunately for such believers, there are entire clubs which go about doing this. Each club tries to "answer" another's by producing complex patterns in fields. They have admitted it publicly on numerous occasions.
Another hoax has been the so-called "historical" artefact which purports to show aliens having visited the Earth hundreds of years ago. They have all been positively exposed as fakes or given normal historical interpretations in virtually all cases.
(7) Reports of alien autopsies, abductions or close encounters are usually made using unconventional methods and by individuals of questionable credentials or by television shows with about as much credibility or integrity as "The National Enquirer". Numerous credible experts who examine these reports pronounce repeatedly that they are hoaxes but that's not what gets reported. Only the initial, un-investigated report is what is given because only that is sensational. When the "Oh, is that all it was. Gee, do I look foolish" truth is made public it is no longer "news-worthy".
(8) It is not reasonable to simply "believe" that something is true without proof even if one would like it to be so. It is not reasonable to accept something as "evidence" or "proof" unless it is unambiguous. It is not reasonable to accept "personal experience" as proof because of the unreliable nature of human perception. It is not reasonable to believe in something that contradicts the known laws of Physics. Even if there are some unknown technologies that can overcome any of the obstacles to interstellar travel, the known laws of Physics must still apply. Even if all the contradictions in UFO reports are disregarded, as well as the reliability of human perception, the limit of the speed of light plus the enormous time and resources required to travel the vast interstellar distances which are involved all preclude the possibility of such travel with virtual certainty. Ignorance of the sciences of Physics, Biology, Astronomy, Engineering, Geology and Cosmology and the social studies of Psychology and History together with the intrinsic human qualities of suggestibility, gullibility and psychological need are responsible for all the aliens or UFOs ever "seen".