While many people point to Drake's equation when discussing the possibility of extraterrestrial civilisations, I find the Fermi paradox just as important - worth a read if you're still hung up on the idea of "UFOs".
http://en.wikipedia.org/wiki/Drake%27s_equation
http://en.wikipedia.org/wiki/Fermi_paradox
Personally I see the universe as a big soup of physical/chemical interactions. Intelligent life (or local pockets of 'order' to be a bit pedantic) in my view is inevitable through so many simultaneous processes, that is assuming the laws of physics making the processes possible continue to exist in a time-frame reasonable for such order to come about.
Why haven't we seen any evidence elsewhere? Apart from the huge distances involved and the other points covered in the Fermi paradox, there always has to be a 'first' civilisation. I'm sure that sounds obvious, but what if we are the first? We wouldn't see any evidence but would have to wait around for the 'second', etc.
Now I'm not saying this is what's happening, but it's a possible scenario.
What I don't believe in is 'aliens', seemingly so technologically sophisticated that they can navigate across the galaxy without effort, coming in and newbie crash-landing into Earth and/or hiding outside people's windows and/or hovering and disappearing for no reason. So silly I can't understand why there are serious UFO discussions in the first place.