When posing the questions of why/how we exist in a scientific context, the questions can be posed/organized/framed in many ways, but they typically have the same layout. The cosmos can be described as a series of scales and explanatory mechanisms roughly corresponding to the layout of the Drake equation. The possibility of life is can be seen as the product of a series of possibilities:
This weekend's Origins Symposium at ASU is laid out in this way, with cosmologists speaking at the beginning leading to biologists and ending with discussions of human culture. This also corresponds roughly to Dennett's "grades of possibility" as well as the typical ladder of reductionism (philosophy -> physics -> chemistry -> biology -> social science -> philosophy -> just kidding).
Associated with different layers are different formulations of the anthropic principle. When listening to what I would usually consider one of the less interesting and more mundane layers [the formation of earth-like planets -- just a bunch of dirt flying around], I found one of the more interesting formulations of the anthropic principle. It is turning out that earth-like planets may be extremely rare.
If earth is unlikely, then why do we exist? Because we are here to ask that question, of course! DON'T assume, as cosmologists have historically done, that our region of reality/space is typical [the mediocrity principle]! We can only assume, at best, that it is typical of places like ours! It sounds a little circular, but we now have evidence of some formulations of the anthropic principle against the mediocrity principle. I haven't read the article yet, but the cover of this month's SciAm reads "DARK ENERGY Does it really exist? Or does Earth occupy a very unusual place in the universe?"
I suspect that evidence in one layer will spread the anthropic principle through the cosmological pyramid, weakening the mediocrity principle of other layers as well [In case you can't tell, I've been reading Darwin's Dangerous Idea]. Maybe we are special after all.