if we assume that the universe is infinite, then there is an infinite number of planets. if we look at our planet and say that it is the only one with life, then we are saying that our planet is 1/infinite. That isn't possible. There's no such thing as 1/infinite. Even if there were a billion planets with life, we'd be saying that these are 1,000,000,000/infinite. Still impossible. The only sensible conclusion is that there is an infinite number of planets with life, because only then does it make sense; infinite/infinite = 1 = an existing answer
so yeah
HHGTTG, no?
And, of course, there's that initial assumption which I may want to pick you up on...
I read somewhere that the conditions that allowed Earth to sustain life are extremely rare to have occurred together, but even "extremely rare" when applied to the billions and billions of universe means there are thousands of galaxies that, statistically, should have life-sustaining planets. And we've discovered planets (relatively) close that come close to those conditions.
We can't say for sure anything about the nature of the other life, whether they're sentient beings or just bacteria, but it's pretty ignorant to think we're the only life in the universe. And I think you'll find most people here agree with that.
Anthropic principle. Just because there are relatively few planets that can support life like us, doesn't mean that a different kind of life that can stand different conditions hasn't appeared elsewhere. Not sure if that's what you were saying already or not. I agree with the second paragraph.