First of all, writing is fun! I write to bring my readers enjoyment - and myself as well.
Secondly, it's good to get your views and ideas out - and maybe cause a ripple or a stir in the great waters of the world! (Or the little waters, too)
Thirdly, I started writing even before I could write - drawing pictures and saying the words out loud. It's been a long time now, and I still haven't run out of things to write.