I feel like, at least for myself, working out triggers something in the brain that makes me less satisfied with how I look and feel.
When I wasn't working out regularly I was quite content with my physical appearance. All the years I've spent working out off and on it always seems like the more I workout the less content I am.
It feels a bit like hitting a goal and then moving the bar. You're all hyped up about a new PR for a day or two, and then it quickly turns to "fuck that number, I can do better!", and the cycle of the self improvement grind continues.
Once you figure out you can lift heavy things you just want to lift even HEAVIER things.