[perdunov]

Because it's incredibly improbable that we are alone given how much stuff there is in the Universe (something about 10^23 stars) and how prevalent are the chemicals we consist of.

[backlava]

But 10^23 could be vanishingly small compared to what's in the denominator of that probability. How do you have any idea what the odds are on the formation of life?

[perdunov]

It's too hard of a question. Of course, this is just an intuition. No formal proof or calculation implied.