There is no denying that people see strange objects in the night sky on a regular basis. Someone recently told me a story of a object he had seen with a group of people that reportedly would move in one direction, stop, then completely change direction and continue moving along. Even I have seen a UFO. I was just starting out as an amateur astronomer. It was twilight, and to my amazement I saw a bright object hovering over a nearby lake. The object was flashing white red and blue. I was completely taken aback and at a complete loss for an explanation.
There countless stories like the ones above. Still, I have heard even dedicated UFO proponents admit that probably 98% (or pick your own number) of sightings and encounters are hoaxes or misidentified normal phenomena. It is those 2% of sightings that we can't come up with a explanation for, that they believe represent the real encounters.
The problem with this argument is that you are moving from an unknown (hence unidentified flying object) to an answer (aliens exist) without any further evidence. Scientists work with unknowns all the time. They may have a hypothesis that they are trying to gather evidence for, but without that evidence, it remains an unknown. Law enforcement does the same thing. If a case can't be solved, they simply label it unsolved. They don't assume that aliens did it.
With that said, I want to look to see if there is any reason that this small percentage of cases might go unsolved if aliens are not visiting our planet. Some of them probably are hoaxes or hallucinations. As Randall Munroe of xkcd shows above, it doesn't take many people misunderstanding something they seen in the night sky to make a lot of UFO stories. In may other cases, we just don't have enough information to find any answer.
If you want to say aliens exist you need evidence. UFOs, even those cases that are never solved, just don't cut it. Take my story above. I see no fundamental difference between mine and many others out there, except I found out what it was. I stared at the object over the lake for hours off and on. As it slowly rose in the sky, its colors settled down into the bright familiar glow of the star Sirius. The wild colors were probably caused by a nearby fire or other air pollutants. Still if I had reported it as a UFO, without some key details, no one could have ever proven what that object was. I doubt we will ever solve all UFO cases but that doesn't bother me one bit. What gets me going is the trill of finding an answer, one I can explain and defend, and growing my understanding of how the universe really is.