Another reason is that the lighter my base pack gets, the better I walk, AND the more fun stuff I can carry. I like to bring toys and better food. I don't feel so bad when I bring the 4 pound pack raft, and Trout cooked with bacon is wonderful.
The last thing is that I am competitive. I usually beat my friends pack weight by pounds and still manage to bring more fun stuff than they do. Frankly, I get a kick out of how amazed they are.