Draco hates Harry for a lot of reasons. The fact that Harry is a hero, for a start. It's hard to compete with that. The fact that Harry is so very good and pure. The way he's friendly to almost everybody. The way he says sorry if he runs into someone in the corridor instead of glaring at them.
Mostly it's the way everyone adores him. Most people agree that he's attractive; most people have some sort of a crush on him. It's the way that people look at him and smile because when he's there, their day has brightened somehow.
It's the way Hermione gazes at him when he's not looking, and the way she casually touches him. It's the way Harry doesn't realise how crazy she is about him, the way he's so blind to her obvious admiration.
Draco hates Harry for a lot of reasons, but he thinks that one might be the most important.