I get the feeling that the girl is simply happy and young and the protagonist is older and jaded with life. He wants what she has. The happiness that eludes him. So the very thing that has drawn him to her, her innocent joy, has become suspect. So he begins to cause her pain to see if her happiness is real--Having lost all faith in anything so pure, he doubts her and suspects her happiness isn\'t real--but masking the pain that, in truth, he tries to hide or tries to take from others. He\'s projecting himself on to her. He hates himself and in the end, hates her.
I get the feeling that the girl is simply happy and young and the protagonist is older and jaded with life. He wants what she has. The happiness that eludes him. So the very thing that has drawn him to her, her innocent joy, has become suspect. So he begins to cause her pain to see if her happiness is real--Having lost all faith in anything so pure, he doubts her and suspects her happiness isn\'t real--but masking the pain that, in truth, he tries to hide or tries to take from others. He\'s projecting himself on to her. He hates himself and in the end, hates her.