I have no idea. I don't remember a time when I didn't read. I've seen pictures of me as a small child pretending to read the dictionary, so I imagine that as soon as I was able to actually read I began reading the books my parents used to read to me.
I guess the answer lies in my parents, more specifically my dad. He loves books and there were always books around when I grew up. Because of him, I suspect that somewhere along the line I must've formed the world view that that's what you're supposed to do with your free time.