I'm pretty sure it's because ML had suppressed his feelings. ML only remembered the bad which is MC leaving and realistically when we've been hurt we only remember the bad. Plus, think about it, it's been 10 years and that's enough time to deeply suppress things you don't want to think about. Yet, the more time ML and MC spent tgt and the more ML saw glimpses of what MC had been through the more those buried feelings are starting to slowly flow back to the surface.
Because in his eyes, MC abandoned him? Yeongwoon suddenly cut him off after Seo-in was forced to kiss his adoptive brother and implied being his friend was a burden. The only person Seo-in ever truly liked and the only person who treated him with love then left him and ran away. Not to mention he probably has some sort of personality disorder that makes it very hard for him to put that behind
he's hurt by yeongwoon saying those things to him and him leaving without giving me a reason, so it feels like abandonment. and it especially hurt because he did like him and he did all that just for yeongwoon
After reading the backstory im confused why the ml didn’t treat the mc nicely when they met each other again..