Because thats what happens when you fall in love, when you give all of yourself to someone, you start depending on them, and at the slightest withdrawal of interest, you fall apart. Because when you're in love, all you want is for the other person to be in love too. And it hurts when they're not. Because you're thinking 'how dare someone take my heart forgranted ?' and 'how dare they accept my heart without any prewarning of the slightest possibility that they could fall out of love ?' But you're lacking the realization that you're a hypocrite, because you, too, have done that a million times over, breaking hearts without second thought.