It seems to me that, given that we know from our own experience that life CAN exist (at least in this particular universe), given the immensity of our universe, and given the vast number of other universes that exist, it seems unlikely to me that Earth is the only planet with life. (I know; that is the same old argument, but I find it's logic is quite compelling.)
I always come back to this question - would we even necessarily be able to recognize other life forms, if we stumbled upon them? Other life forms might or might not be intelligent. Their lifespans might be much shorter or longer than ours. They might not be carbon-based. So many things could be so very different that some of our most basic assumptions concerning the nature of of "life" could affect our ability to recognize it when we did encounter it.
And then this question - who is to say that we actually understand what "life" is and/or what part of it we represent? Humans could possibly be the equivalent of enzymes or something in a vastly larger organism. Whole new, unsuspected realms of science keep being discovered (e.g., quantum physics, dark matter, etc.), so any confidence we have that we truly understand things is already on shaky ground.
And where does that leave me on the question of whether there is alien life? I would have to say that, for all practical purposes at this point in time, we are alone, but to hedge our bets on the intelligent side, it would probably be wise to assume that alien life does (or can) exist...if only for the moderating influence of the humility inherent that assumption.