Can some pls explain their past i am a bit confused and why does the brown haired mc and b...

This is all my opinion because some of this is still speculative.
Ml doesn’t hate him but felt like mc abandoned him. He didn’t know that mc was being bullied by mls brother or that mc ran away after his dad fucked him over. So ml is just going through hard core abandonment issues.
Mc never reached out because he didn’t realize how important he was to ml. He saw ml? kissing someone else in hs so he figured ml didn’t care about him.
There are theories that the person mc saw kissing was actually mls brother since they look so alike. Hopefully it’s cleared up in this season. Either way mls brother is a major ass.

according to what we've been reading, yeongwoon saw seoin kissing his brother, it's implicit but he probably felt betrayed because they had sort of a 'relationship' where they kissed and he was in love with him. we also know that before seoin went abroad, his brothers and friends withheld and hit yeongwoon so he couldn't go to the airport to say goodbye, the mother paid yeongwoon's dad after that beating and probably that was when yeongwoon ran away, and when seoin returns after three months and tries to look for him, he had already fled. another memory that comes up constantly is yeongwoon telling seoin that he is tired of being his friend (can't remember exactly), but no memory is linear and you just have to keep reading for the puzzle to be assembled, overall it's a big misunderstanding and bad communication that was probably caused by his brother and their bad relationship