Do you ever ask why the wind blew a piece of leaf right on your nose? It is merely the luck of chance that made that leaf fall right on your nose. It was one of the permutations of possibilities of gravitation, the tree position, the leaf weight, the wind direction, the humidity and your position that actually came to fruition.

In the same way, the formation of life is one of the permutations of many variables the weave different possibilities.

Even a life of a rat is determined by the same variables but you would never ask why a nameless faceless existenceless rat was born. It is the same thing but you would rather glorify your birth as a human being due to the extra ability of language which makes your thoughts wander past the boundaries of reality. Truth be told, there is not right answer but it's good to stay firm on ground realities and not bother too much about things that are way out there.