Chess is mental torture.
This quote from Garry Kasparov is more or less spot on. When you play chess properly, which means treating it as a matter of life or death, it can be a very painful experience. One tiny slip and the product of many hours work can be wasted. To drive the point home here’s a video of Kasparov’s reactions making some mistakes:
Putting yourself through this day after day, for example in a tournament, is also exhausting. Many people only discover this when they actually try it.
Why do they do it and come back for more? Well there’s nothing quite like it and people learn a lot about themselves. There’s also the challenge to get better which often means improving our abilities in a way the extends beyond the checkered board.
How should someone cope with losses? Well a good start is to have a relaxed attitude and the ability to smile about it. I always got the impression that Kasparov put incredible pressure on himself and it must have been a relief when he decided to retire. Magnus Carlsen, on the other hand, comes across as being far better equipped to cope. So my guess is that he’s going to be playing for a long, long time.