Why do I feel mentally worse when I stop exercising?

When I look back on my life over the last 44 years, the absolute worst years of my life mentally always had a correlation to how I was doing physically. And so all through high school, even though I was physically inclined and actually kind of physically gifted, doing push-ups at home and whatnot, I just wasn't very active. I had so much energy and it was all getting used in the wrong way. And so when I transitioned out of that phase of life into more maturity around 18 or 19, I started rock climbing. So I started rock climbing for three or four years. I worked up to probably V10, V11, V12 in bouldering, somewhere in that range, both outdoor and indoor, and was doing really well, and then got injured, stopped exercising. And I actually can see in hindsight I just started to go downhill. And then I joined the church. And shortly after joining the church I realized that I needed something positive to do with my time and energy. So I stopped drinking and went to the gym, started weightlifting. And so over the last 20 odd years or so I pretty much have been physically fit that whole time, doing something physically. But even I can see there were periods of time where I wasn't doing a lot physically and it really took a toll on me mentally. And so I love to extol the virtues and the benefits of lifting weights, because I've seen it be such a profound help in my life, both for my mental and physical development and growth and overall well-being.