Very polite. We Koreans don't really say often. But when we say that we really mean it I think. 친구가 돼줘서(되어 주어서) 고마워. Thank you for being my friend. 친구가 돼줘서 기뻐. I am happy that you become my friend for me. And then, we will probably say. 그런 소리 하지마. 나도 네가 내 친구가 되어줘서 고마워(or 나도 네 친구가 되어서 기뻐) Don't mention it. I really feel grateful that you become my friend(or I am very happy as well that you become my friend). Normally when we say thanks btw friends, we do say 'Hey friends don't need to say 'thanks'. Because friends do help each other, it supposed to be' '에이 친구끼리는 고맙다고 하는 거 아니야. 친구끼리 원래 서로 돕고 그러는 거잖아' Does it help you out a bit? :) Hope you have a great day.

에이 친구끼리는 고맙다고 하는 거 아니야. 친구끼리 원래 서로 돕고 그러는 거잖아^^ komawoyo