I lift because I enjoy it. A lot. I enjoy the feeling of my eyes about to pop out in a squat. Or the heat of the blood rushing to my face in a deadlift. It gives me something to do and it gives me something to be proud of. While I wouldn't brag to an 800 lb deadlifter that I have a 400+ lb deadlift(and I don't brag to my friends unless they want to hear about it), I get a sense of accomplishment.
Other than that, I've always wanted to be stronger. I used to tell my parents and my brother that my brother would be taller than I, but I'd be stronger. He's 2 inches taller than I am and I am stronger than he is. Then of course, to be bigger. It took a long time for me to really WANT to be bigger. Try over a year. I read about those abs I used to not have when I played video games all day, ate pizza rolls and drank countless cans of pop each day. I went low carb, low calorie, low this and low that... I finally realized the difference between now and then. I lift. Fat? Not worried about it.(well, yes, but not so severely. It's easy to lose weight. Hard to gain) I just want to be bigger now. (Sorry about the story telling)