Most people only aim to become famous or get money for their own scholarship. So when they lose, it just tells them in their own head that they are doing bad for the next tourney coming up. So they think they suck, that they will not win.
The Pokemon TCG is a mind game mostly, it's about what people think and people's actions. Not about which deck is good or not.
Little kids like that have attitudes like that all the time. They always think they are automatically the so called ,"World Champion." But they are not, they need to chill down their acts. If they want to be a really good player, It takes time. More practice and decent combos for which deck.
I see all those juniors throw tantrum all the time. Another thing is probably why they haven't played a good player in league. But when in City Championships or something like that, they get 6-0'd by the top in the state. Or the kids get too cocky to win, they get carried away, then make a bunch of misplays.
Too many people get carried away before winning a match, that they make a misplay and then lose the game. I have went through this situation and I know it, and that situation mad me mad also. I know how these kids get mad, I seen their reactions.