Well, I don't believe there is a particular reason. Randomness. We are here by pure chance. And it may very well be that there are other "coincidences" like us in the universe. But the universe is so large, that the chances we ever come across life-forms on other planets are very very slim.
Or, you can believe that there is a purpose to everything, and in that case, good luck...because I think your questions will ever go unanswered.
I am no bashing religions, beliefs, fate and such. But for me, it's just not there. After all, we only are the sum of all our experiences in life, and so far this is what it sums up to, for me, in this department.
The way I see it, life is the result of a complex equation with millions of variables. And that result is only attainable when all those individual variables attain a certain value. For a quick analogy, let's use a deck of cards. You've got 52 cards and you're drawing 4 of them. To get "life", you need 4 aces. So in this case, the probability of "life" would be (4*3*2*1)/(52*51*50*49), or 24 chances in 6'497'400, in other words very very very unlikely. And this is just with 52 possible cards, and drawing 4. Now, life on Earth is more like a deck of a few hundred million cards, and you're drawing around a million cards. A "miracle" might some say. To me, it's just probability and randomness.
PS: Hope my math is correct, it is after all 6AM xD
Cheers everyone,
PLUR