Because he went through it before in 08/09 probably. When you've survived it once you know you'll survive it again.He's been in the public eye now with the glare of publicity for 18 years. Since 2012, he has managed his public brand incredibly well. I find it hard to believe he's laughing while talking about the most bitter defeat any tennis player has ever experienced in pro tennis. But he doesn't seem phony here, seems legit. That is utterly amazing!
We don't know if he'll regard it as his worst defeat when he's looking back, maybe he's rationalised it somehow.
But I said on another thread, he'll have to think about his girls, who are now at the age where they'll understand what's happening to some extent. If they start to get worried about whether he wins or loses then that could affect things.