
I think its to give Haebom's bully a taste of his own medicine.... (๑•ㅂ•)و✧

Maybe it comes from a mutual misunderstanding, Taesung who hurt Haebom when they were little might think that any kind of interaction with him now will probably hurt him again since it looks like Haebom is not the outgoing type and wont aproach Taesung either... While maintaining their distance outside their home they can keep to themselves their problems. Haebom may think Taesung doesn't like him much and the same goes for Taesung, but they are changing...
That's the only explanation I get from what little we have until now!

yeah, i get what you are saying but the fact that he 'pretended' to spend haebum's money in front of his friends rubs me in the wrong way. maybe he didn't want to approach haebum that openly in public for some reason (the mutual misunderstanding, but why is he sitting with him in lunch now?) so he didn't give it back immediately after getting it from the bully but his action after didn't make any sense. oh well. ╮( ̄▽ ̄)╭

Hm, maybe he doesn't want to share Haebom with his friends, especially when their relationship is still like this: if he'd let his friends know he returned the money, his friends would want to know about his relationship with Haebom and eventually they'd want to befriend him... and imagine if Haebom started getting along better with Taesung's friends when he still barely exchanges a word with Taesung himself... the jealousy!
i still don't get the motivation behind taesung's actions though. i mean, if he was planning to give back the money, why didn't he do it immediately? why did he pretend to spend the birthday money to treat his friends??