Exercise is also something you can "get into". For example, you force yourself to go to the gym a few times, or to go running a few times. And then you start beating your previous records, at which point you get more ambitious. You try to research the best supplements on the web, or the best pull-up bar for at home, and before you know it, you are transforming into a "gym bro" without previously intending to. It's remarkably easy to get into various obsessions, even if they are entirely healthy.