Maybe he's trying to protect her from some danger or threat. Or it could be that he's worried about how others will react to their relationship.
Well, one reason could be that he's involved in a complicated situation and doesn't want to drag her into it. Another possibility is that he's not sure about his own feelings yet and wants to keep things quiet until he figures it out. It might also be that there are external forces, like family or societal pressure, that make him keep the relationship a secret.
The hero might keep the heroine a secret because he has a past that could affect their future together. Or perhaps he's promised someone else and doesn't want to break that commitment. Maybe he's concerned about her reputation and wants to wait for the right time to reveal their connection.
Maybe she's scared of commitment or has trust issues. Or perhaps she feels like the hero is too intense or overwhelming for her.
Perhaps the hero hides her out of jealousy or possessiveness. The result might be a lot of drama and misunderstandings before they finally work things out.
Perhaps Dumbledore foresaw some sort of prophecy that was related to Harry and his parents. In order to fulfill the prophecy in the right way or to avoid a greater catastrophe, he kept Harry from his parents. He was always thinking about the greater good of the wizarding world and might have believed this was a necessary step.
One possible reason could be that the hero has a bad reputation in the society they live in. Maybe he is known for being a playboy or having a cruel streak, which makes the heroine despise him.
One possible reason could be a misunderstanding. Maybe the hero misinterprets the heroine's actions or words and lashes out with an insult.
One possible reason could be a misunderstanding. Maybe there was some miscommunication in the past that led the hero to form a negative opinion of the heroine. For example, he might have overheard something that was misinterpreted.
One possible reason could be his Slytherin pride and the long - standing rivalry between Gryffindor and Slytherin. He might have said something mean to Hermione in the heat of that rivalry. Another reason could be jealousy. Maybe Hermione is achieving more academically or getting more recognition, and Draco, being a proud pure - blood, can't stand it.
One reason could be that he thinks they are too unrealistic. Heroine novels often have elements of fantasy and over - the - top romance, which might not appeal to his more practical or down - to - earth nature.
One possible reason could be that he has some sort of hidden agenda. Maybe he wants to use Hermione as a bargaining chip later. Another reason might be that there is some magic or force compelling him to do so against his will. It could also be that he has a momentary change of heart, perhaps seeing something in Hermione that reminds him of a past acquaintance or a long - lost bit of humanity within himself.
One possible reason could be that she has seen the negative effects of love in the lives of those around her, like a broken family or friends who were hurt in relationships.