Simple, easy to answer? Not really.
The answer I always give (and seems to be widely accepted) is because I love to learn. Books allow you to do this without having to leave the comfort of your own home (which for someone with social anxiety- is a great thing).
But. That's not the answer I want to give.
The real answer is so much more than just my love of learning.
Books are essential to who I am (cheesy, I know). I walk into a book store or library and I know that I am home. I could spend hours wandering the shelves and isles Books give me a sense of peace- when I am reading or spending time in a library I know that I am doing something I love. I open a book and get lost in the words, characters, pages, settings, and feelings (even if I dislike the book, I still love the time I spend with it). I get to travel to far-off places, places that don't exist, past events, potential future events, current events. I get to feel love, anger, hate, revenge, sadness, fear, jealously, and every emotion in between. All because of words on a page.