J-Wo hits the nail on the head.
If we are talking about UFO's simply as unidentified flying objects, then of course they exist. I saw an unidentified small mammal the other day. It went past too quickly for me to say with any certainty whether it was a weasel or a stoat.
But if we are linking UFO's to extra-terrestrial life (as very many do), then on present "evidence" the making of that link is a step too far.
Leaving aside hoaxers, genuine mistakes, optical illusions or whatever, a significant number of these objects seem to appear in remote places, often near air force bases or other military installations. There is footage of flying saucer tests that were being carried out as long ago as the 1940s. If I was in military top brass, I'd much rather people believed they'd seen little green men than top secret military hardware.
None of this is to say there aren't little green men out there. But like any other claim, it requires proof. And like so much of this stuff - UFO's, Sasquatch, Nessie etc - the proof is always lacking, and what there is tends to rely heavily on hearsay, circumstantial evidence or self-serving evidence.
Take the OP as an example. I don't necessarily doubt what SB says s/he saw, but a clear inference is being drawn (especially from his/her second post) that what was seen was - or might well have been - extra terrestrial. And there is no proof of that. At best, all SB can say is that s/he saw something funny in the sky. That's it. It can be theorised that it was not of this world, but in the absence of any firm evidence to back up that hypothesis, such a statement is no more valid than me saying I think that there might be a goblin behind the door. It's never there when I open the door, but perhaps that's because it hears me coming and runs off.
Regards,
Peter