I can see how you could call it a habit. Back in elementary school we had the Accelerated Reader program where it was, more or less, a competition to see who could read the most books. And reading was the only thing worth doing as a kid when I went shopping with my mom and she went to one of the many Thrift Stores around here.
And in eleventh grade I always had a book in my hand, as noted by my study hall teacher. Once I really get into a reading sort of mood, it becomes part habit, but the pleasure's always there. School kept me from reading how much I wanted to. I'd start books that I generally enjoyed, such as The Road and then stop because I knew I'd derive more pleasure from it if I could give it more of my attention. For a while, I stopped reading entirely, then I got back to reading starting by re-reading one of the few books I own (I'm very particular about what I own; I'm generally a library person). And re-reading that book was entirely for pleasure.
There is a third reason, though, as people have already stated. To write, you must read. The two go hand in hand.