I, personally have a problem borrowing books. (Before I start, I want to say I only hold myself to this, I don't judge anyone for borrowing books nor do I think libraries should go away or anything. I think they're good)
On some random message board someone said something like "How would you like it if you worked really hard on something, got it published, and then people just traded your work around the rest of your life" (It was worded better, of course.) It sort of hit home for me and made me feel guilty.... so I only buy books now.