He has always been by her side to protect her since she was a child. She willfully thought that he should treat her well, just like a brother who loves his sister.
But that's not what he thought. He vowed to marry her as his wife from a very young age. He began to protect her everywhere. Every time she got into trouble, he helped her deal with the aftermath.
One day she told him that she was in love with someone else. He began to wonder if he was so protective of her that she ignored his existence. He began to learn to let go of her hand.