Many interesting and beautifully put explanations above, but I'll see if I can add anything to it.
I guess I shoot pictures because that is who and what I am. To create an image which reflects how I see the world rather than how we just look at it gives me satisfaction beyond explanation. For a few seconds when I first see a picture I have created when I'm out in the street and has been published I feel that I exist and have some purpose. The feeling fades over the next 24 hours, but for that short period I am me and I am real.
I have yet to capture the perfect picture for myself and maybe I never will. But I will keep trying until the grains of sand run out and all that is left of me is the prints I have created. Hopefully one day someone will look at a print and wonder about the subject and the unknown photographer, but I am not as important as the picture.
I also feel guilty at times that I have dumped well paid jobs to chase the light, sometimes to the point of a "starving artist" which is not good with a wife and son. But my son is proud his father is a photographer and my wife loves me enough to work in order to support us and any fee's I make give us those little treats just every now and again.