Dib finding out Dwicky's been lying to him in "Vindicated!" When you see how he joyfully and rather innocently reacts to someone extending a measure of kindness and tolerance towards him, it starts to kind of hit you just how ridiculously YOUNG the poor kid is, and when his "ally" is taken from him it becomes almost impossible not to see just how thankless and lonely a life he lives in trying to save his own people from extinction.
Dib: You don't believe me! You... (builds towards rage, then regresses to pleading, and winds up just staring at the ground in defeat) I thought you were different.
Dwicky: Look, I know you think I lied, and I—I did, but I was only trying to help.
Dib (bitter): Isn't this just great?! While you waste my time, Earth gets one step closer to being taken over! That's my problem right there! My problem is that the human race seems to want to be destroyed!