Poker can be an extremely frustrating game.
I can vividly remember certain moments; flopping top two pair, only to see my opponent catch runner-runner to beat me with an improbable straight. Folding a small pocket pair to a solid raise, just to agonizingly see the matching card appear right in the window of the flop. Getting it all-in pre-flop with an over pair, only to have your opponent hit a set on the turn. Chasing nut flushes to the river in vain, ultimately losing a potentially monstrous pot.
And just when you feel that you have figured out this game, poker karma comes back to bite you, humbly reminding you that you still have a lot to learn.