When this doesn't happen what do you do?
You generally do not go to the gym and fail, exercising works more or less the same for almost everyone, you get good hormones, you feel good.
Socialising, on the other hand, is entirely different. Some people thrive in it, some people feel much more dread afterwards.
No, no, no, it's absolutely not the same, OMG, nothing alike. "I dont want to go to the gym today" isn't the kind of profound, all encompassing, and existential dread that attempting to organize a social event is. Especially when you push yourself to organize and it doesn't work out, which has happened to me before. Those feelings are legitimately nothing alike, the fact someone is comparing the two is wild to me.
I do still need to try to overcome it and get over it, but it's not even as remotely as simple as you claim.