"UFO" is a major misnomer. Not all "UFOs" are "objects" and not all are "flying" (e.g. light diffraction, vapour trails, electrical activity and other phenomena). To leap to the conclusion that such things are proof of little green men is an even bigger fallacy -- so big I won't even deal with it.
Of course, doubting the efficacy/veracity of "UFO" sightings being proof of alien existence is different to doubting the existence of aliens full stop. To put it mildly, the universe is a formidably vast place; vast in space and vast in time.
Something to chew on: in "Cosmos", Carl Sagan conservatively estimated that there are perhaps 10^22 planets in the universe (ten billion trillion). That was thirty years ago; due to more recent sky mapping and other discoveries, that number can be revised upwards, today.
We also know that there are organic compounds throughout the cosmos. The chances of life arising just once, on one planet, in one obscure region of the universe, seems, well, remote. But we still don't know. We've created organic compounds in the lab, but we have never seen them begin to self-organize and replicate -- the critical process for life. If and when we discover this, it will be one of the biggest breakthroughs in modern science, and we will then be better equipped to understand how common, or how rare, life in the cosmos really is.
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The world is not to be divided into sheep and goats … The living world is a continuum in each and every one of its aspects. -- Alfred Kinsey
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