It wasn't that he needed to feel better, specifically, although kissing Ginny did accomplish that. Harry wanted to do something good, make someone happy, after everything that he'd been thinking about and feeling today. Telling Ginny about his day wasn't going to accomplish that, it would just make her angry, especially if he told her about Zach. Harry didn't think that was necessary.
His breath caught at the feeling of her teeth, and he deepened the kiss, opening his mouth to let her tongue in, moving his own against hers. His arms went around her tightly, one hand curling in her shirt, and he let himself get lost in it, instead of holding back the way he usually did. He wanted to do something good for her, yes, but he was drawn to how good it felt, too. Maybe he did need it.