[There's something comfortably familiar about all of this: a prison ruled by mysterious powers, gambling, hideous murder. It's like the City all over again, but without the friends and a sense of purpose.
So, actually, it's kind of depressing.
Since Chekov doesn't know how to be depressed (mopey is not the same as depressed), he's trying to enjoy himself. Trying and succeeding, even! Winning might be pointless since there's no monetary gain, but lights and noises and buttons are all very entertaining. It's a fine opportunity to exercise his computational abilities, too; using statistics and probabilities to an advantage when gambling is simple enough, and determining the optimal size of a series of bets in games of chance is even more exciting.
He wanders around from game to game, alternating between snacking, informing anyone who will listen about Russia's historical contributions to the world of gambling, and placing his bets with a little too much confidence. Subtle he is not.]