When I was an intern, I worked overtime when there were big projects to do. The reason was that it was my responsibility to get them done. I also worked overtime when it was optional, because I liked the extra money, I liked the work, I liked my coworkers, and if I still had time to go to the gym and take care of my apartment and myself, that was all I needed.
Of course, this was a four month position, after which I went back to classes and the overwhelming amount of free time I had between and after them.
If I worked an hourly job again, full time for a long period of employment, I'd probably still do OT here and there, but only if it didn't interfere with my other interests and only if I enjoyed it. The money is nice, but at times, there are better things to do.