@Ana1234 - I always think of it in terms of what it can do for me psychologically. I think kinds of "good addiction" can go under this kind of feedback. So, if you exercise in a way that releases endorphins and makes you feel good, you're more likely to want to do it again and again. Then, the more you do, the more you'll want to do.
Of course, it only works to a certain point though, because most people don't end up exercising all day every day. But I guess that means that addiction to drugs probably works through a positive feedback loop as well. In that case you aren't able to stabilize it because the reaction to the drugs gets weaker, so you keep wanting to take more and more of them.