The gym, once you get into it, can be incredibly addicting. I'm not as consistent as I should be, but there have been times I've spent 3 hours at a time. I go through similar cycles with running as well. There's just an incredible high to these activities.

It's worth noting that a great deal of the high depends on biology. Some human beings produce high quantities of endorphins under muscular strain. Others don't. Not everyone gets the same high, and some people may not get high at all. It's the same from eating peppers, for example. For those that don't get the high, the gym can feel like a slog.