Why are chess tournaments casual when waiting for the games to start and serious when they play?
Because chess involves a lot of mental strain, they maintain
their serious attitude for the games only, and relax and calm down
when not involved so to rest their minds.
Similarly to other sports, players will remain casual and even
friendly with other players, keeping the game attitude for the
game, because they are only opponents on the field or on the table,
not in real life.