Imagine Harry Osborn finding out you‘re dating Peter Parker.
Harry was a liar, and he knew that. But he had grown up with a lot of time to himself and a mirror, so he knew he had grown quite good at it. So when he had said that he didn’t give a shit, it was all a facade. And it really sucked that he felt the need to do that.
But you, the only person that he really cared about other than himself, had ended up dating someone that he used to trust. Parker. Peter Parker, the one person that he now hated more than anything in the world. He had merely disliked Parker for his reluctance to help on ... certain matters, but now he downright hated him.
He stared at you as you looked upset about his reaction to your news. Like you had thought that he would be happy for you, when he clearly had feelings for you? He might not have been very forthcoming with his emotions, he certainly didn’t shove them in your face but he thought he made it clear that he was waiting for you to share his feelings.
And he had to learn to really not give a shit.