I think what most people don't believe is that there are humanoids on other planets. Think of the big variety we have in animals and how many circumstances and (to us) random decisions by nature made us how we are. two arms, two legs, 10 fingers and toes, "walking"... This all happened because of the right fitting of countless variables. This is the problem when people think of aliens and they make them look close to us.
But I definitely believe there are more planets in the universe that have similar circumstances to earth so there could be life (if it's intelligent or not).
Another problem is that we suggest that "life" has to be similar to what we call life which could be entirely false, of course :)
Perhaps even aliens came once to earth which I somehow just "believe" given that so many religions and ancient tombs and so on concentrate on stars and sk/heaven - all this reaching for the stars (tower of babylon, pyramids) makes me believe that we once came in touch with higher beings.
