Every point one side or the other loses. Every match one or the other loses. When you sign up for a tournament you’re signing up for a loss. Only one person wins a tournament. It’s not gonna be you very often, if ever. After each round, half the field loses. A player in the top 100 in the world will win far fewer matches than she loses. Week after week this goes on. She travels the world taking beatings.
You miss an easy shot, in a big situation. It’s a dagger through your heart. You get no such anguish sitting by the pool or watching a movie with your friends.
In some ways playing tennis is a bit like owning a pet. The more you love the pet, the worse the loss will be when the pet dies. The more you put into the relationship with the pet, the more you get out of it, but the harder the loss is. If you don’t really put much into the pet, don’t really bond with the pet, the loss of the pet won’t be tough to take. No biggie. That’s how it is with tennis. If you don’t put much into the match, don’t prepare that hard, don’t care that much about each point or the outcome of the match, you’ll take the losses well. No biggie. But if you put your heart and soul, your blood, sweat, and tears into your preparation and into the match, a loss will be a big deal.
So you’re stuck with a choice. Care, get the most out of the pursuit, and suffer the losses. Don’t care, get little out of the pursuit, and weather the losses just fine.
Of course, you can also play tennis just for fun. It is possible to go out, play the game, hit the ball, run around, try your best and not care about the outcome. You could even play tournaments with this mindset. But I can’t think of any who do it this way.