I like to play because it's fun to thoroughly explore the (made-up) ins and outs of my PC's personality. Well, that, and it's just outrageously fun to pretend to be a homicidal half-orc who somehow manages to be a productive part of society while still slaughtering bunches and bunches of folks in a heinous and very bloody way.
I like to DM because it's always been my dream to be an author. By DMing, I get to make-believe that I am one for a little while...until those pesky PCs screw everything up AGAIN!
I like to play because it is a great outlet for my imagination, it's fun to get together with friends who have similar interests, and it allows me to become more inspired in my attempts to write my book.