Reasons to hack? Well, most people start hacking cause of the rush. For sure back in the day when I started to learn how to hack and felt really l33t0 with my keyloggers, I got a rush several times. It's just thrilling.
Another reason I found to hack is cause of restrictions at school. You want to install Yahoo Messenger but you don't have the permissions - well screw that. Let's crack that SAM file and get the password so that you can have ADMIN and install yahoo messenger like ya wanna!
After that, the only time I've desired to do a 'hack' is when I'm learning something. I wanted to learn how to take advantage of netbios shares. So I learned how to do it, I browsed around on a few people's harddrives, then never bothered with it again. It's the same deal with exploiting IIS web servers. Just wanted to learn how to do it - do it - then never do it again.
That last reason I gave is a rare one. Now-a-days I don't want to be a hacker. Sure, sometimes I may do a hack, but that's not what I enjoy. I enjoy programming. I say that programmer/hackers (reverse engineering and patch making is an example) is a lot l33ter than exploiter/hackers.