Or, out of the countless other planets, the earth just so happened to be situated in an orbit around the sun that made life possible, and after a long while, life came to be and evolved into what we see to day. Science is amazing.
Richard York Seely II, ever watched Cosmos with Carl Sagan? It's awesome. In one of the episodes he explains the science behind the beginning of life. Once it is properly understood it not only is not surprising, you can see that it must actually be pretty common in the universe. Maybe 1/10000 stars has a life bearing planet, and if that is true the universe is absolutely teeming with life.
To add to the original point, not only is there a 'bio-zone' in a solar system, also within the galaxy it needs to be far enough away from the center (massive black hole) to not be ravished by extreme gravity fluctuations and solar radiation and such. So there is a certain necessary distance from the center of a galaxy as well.
Additionally, the star a planet revolves around needs to have a lot of metals and other heavier elements to provide all the different elements required for complex biochemistry.
But that is just what we 'know' so far. Recently here on earth, we have found life in some really unexpected places such as by volcanic vents on the ocean floor, and in boiling, acidic natural hot springs. Ultimately, who knows what is really required to sustain life. Our own solar system may still hold extraterrestrial life we have yet to discover on places like Io, or perhaps the outer atmosphere of Jupiter.