Why do so many guys hit the gym shortly after a breakup?

I see it all the time. Everywhere. Whether in person or on social media. There are even "guys after breakup" memes. I'm talking about the guys who never cared to go to gym all this time but soon after breakup, things change.

This guy I was friends with (for almost a decade) and I remember him being this sweet guy with a lean build. He was dating someone back then. I was really bad at staying in touch, so I'd be in touch with him on and off over the years. He looked the same for years but when I got back in contact with him 2 years ago, this time I found out he's no longer with her. Not only was he single but he also looked almost unrecognisable. He was now bigger and very muscular. He has this beard too. I was like "Woah, what? Is that my friend?" O. o I was shocked for a moment and asked him when did you start going to the gym? He said, "After my breakup"

He seem very proud of his body and confidence that I didn't see much back then. So, do guys hit the gym to relieve stress and frustration or to gain confidence or there is some other reason?

Why do so many guys hit the gym shortly after a breakup?
Why do so many guys hit the gym shortly after a breakup?
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