I don't know about you, but when I get stressed, overwhelmed, upset, or just a general feeling of being down, the last thing I want to do is exercise. When in reality, the first thing I need to do is get my body moving!
The thing about habit is that sometimes we don't notice that something has become a habit until it's a deeply ingrained habit. And by that point, breaking that not-so-great habit becomes a bit tricky. When you've been running on autopilot, it can be difficult to stop. You need to re-train your brain and your body.