I guess the argument has already been called out, but I also believe in Aliens primarily because I think it is highly conceited to think we are all that live in breath in this universe. I look at the stars at night (there's even millions we can't see with the naked eye BTW) and I think that each one could contain a lifeform on it doing the same thing I am.
Or they could be plotting our worldly destruction in order to construct a hyperspace route.