Agreed only educated guesses. I am not an expert but I would think that it is more than just a dozen factors that contributed to the success of the Earth.
Our position in the Universe and relative position to the center, the fact that we have a moon, Jupiter (a rather large planet sucking up all of the random stuff floating around in the universe that might hit the earth), a fairly young sun (not too large, not to small), Mars might have helped, etc. etc. etc.
A side note: maybe there is a reason why we cannot talk with others in the universe other than distance and time (timing). The likelihood of us actually communicating with another being is pretty nil.
I believe what we know about biology and evolution also support the rarity of life, especially life like modern humans.
We are alone...