They both are just too ill-matched for one another. You saw that initially Guwon pursued her and she was the one that had to open up to him. That means to me that he liked the act of chasing her and courting her more than anything. So when she suddenly became too supportive and too adaptive, feelings started to waver on his end. After her move to the city, she was the one making the time to go see him, she was the one that kept saying “it’s okay” when it really isn’t. Her actions let him feel like he should pursue someone else because her love is “shallow”. Her actions are of a girl in love but it seems forced in a way. During the first meeting chapter, I always felt that she didn’t like him much but he did a single “thoughtful” act and all the sudden she wanted to give him a chance.
Guwon is obviously a sh!tty guy, he’s avoidant and insincere. He claims he wants to break up with her because he “wasn’t sure if he can continue being in a relationship when he might have feelings for someone else”. The reasoning is already really shoddy and just a there to paint him as more terrible than he already is. He never seem to have taken the initiative to see her on the weekends (just because she wanted him to focus on his study), he kept putting off the talk of marriage (I can’t remember the reason if there is one) something Noa has seem to be thinking of a lot meaning he didn’t intend on having a future with her. He made no effort in the later part of their relationship while Noa didn’t push for a reciprocated love.