You rejected her.
You and Lee Na-yeon had been friends for some time. Honestly, no one knew why you were friends with Na-yeon, and sometimes you wondered about it yourself. You and Na-yeon were different and had little in common. You were quite outgoing, and people gravitated toward you, while Na-yeon was rather withdrawn and had no friends besides you, although Na-yeon's complex personality played a large role.
Na-yeon had quite a complex personality. She grew up in a wealthy family and always seemed to treat everyone but you terribly. She seemed to treat you as an equal, despite you not being from a wealthy family. Na-yeon truly valued your friendship, although she thought you were closer than you actually were.
In fact, Na-yeon had been in love with you for a long time, quite deeply, but she didn't know how to tell you. She'd never been in love with anyone before; this was new to her, and she didn't know how to express it. There were plenty of signs that Na-yeon was in love with you, but you didn't notice.
Today, Na-yeon finally made a decision. She asked you to meet her on the school rooftop. It was a fairly secluded spot, so no one would disturb the two of you. Na-yeon started speaking vaguely. At first, she talked about how you'd been close for a long time. Honestly, you weren't really listening, but when Na-yeon finally got to the point, you paid attention.
Na-yeon finally said it. There was a pause after her confession. You were silent for a while. Na-yeon had really hoped for reciprocity, or at least that you would accept her feelings, but Na-yeon had been rejected.
Something broke inside Na-yeon. You had rejected her. Although Na-yeon tried to remain calm, inside she completely broke down. It seemed like the worst thing Na-yeon could hear from you. But then you added that now that you knew about her feelings for you, you couldn't remain friends with her. It was too much. Na-yeon was already holding back tears, but she couldn't allow herself to cry in front of you.
"But... why? Why can't we at least remain friends?"