It might be just me, but he sounded much like someone who is aromantic. That doesn't mean he doesn't care for her, but it's different from actually being romantically interested. Feelings and emotions shouldn't be labeled as "non-mature" or "unserious" either, but that might just be my opinion.
It's totally ok for him to want to be in a relationship with her, of course, if he had been honest from the beginning, but the problem is that while he felt that way, she though their relationship was something totally different because he made it seem that way.
People often separate because of their differences, so don't you think it makes sense that she deserves to be in a relationship where she's loved in the way she needs? Doesn't it also make sense that he deserves to be in a relationship where he doesn't feel the need to overcome overcompensate for something he doesn't feel?
But he should've told her and had a conversation with her but no he hid it expecting her to still love him. And it's also bad enough that he talks about it on the phone
also how can she be selfish if she doesn't know it in the first place
Its not Iseon isn' t honest. Yoomin didn't vocalize her problem or what the conversation she heard.
Also Iseon is clueless and confused about their break up and Yoomin is too late for explaining her problem with Iseon. (realistically, would you be able telepathically recognize someone's problem? No.) she wasn't honest to begin with, instead she kept making hints/clues to Iseon. (pretending to already have a partner to making Iseon come back/keeping the conversation she heard) selfish on her part.
We kept having Yoomin's POV most of the chapters (not trying to offend but we making ourselves putting Yoomin/MC shoes) so mostly here we're biased. (Im not defending Iseon neither Yoomin)
He already told her that he really loved Yoomin and want to make her happy. Yet, Yoomin definition of love is entirely different from Iseon (because she expects), that's why when he explained and questioned her. She got stunned and realized Iseon truly loves her. :)
MC doesn't give Ilson the opportunity to explain himself. She cut off their relationship and expect Ilson to feel the same love she has, when in fact, Ilson has a different kind of love. His love doesn't involve butterflies. It was a mature and serious kind of love that doesn't rely on feelings and emotions.