When you know the loss was genuinely your fault...
Ever know so well that the game went downhill because it was practically your fault? Because you became in the lead at 5 minutes and then do a series of stupid things because you were cocky. You lose the good lead and turn the game around quickly and when you lose, you know its all because of you in the beginning.
Worse part is when your team doesnt flame you. They know its your fault and you can feel their non-existent flaming on you in chat. And you lose the game in guilt knowing what you did wrong that ended so poorly... ward.