We are mentioning many great topics in this thread. For the subject of alternate realities and imagining more of the mirror universe here is a Next Generation mirror universe intro! http://www.youtube.com/watch?v=sZblw7n2t6o
So now we not only have to wonder "why am I me and not someone else?" but also "why am I me and not the me somewhere else?"
That is one of the many interesting philosophical questions about time travel. Themes like predestination, destiny, predetermination, and fate are common. In them we determine how experiences effect an individual in an environment and vice versa.