Responses
I believe it was because the brother is the only one who is gentle and often communicates with him. The bro also pay attention to him the most in the household back then and it seems like they spent quite a lot of time together. The backstory kinda implies these but I can't remember the chapters tho

Im sure he's genius in business..that's why his grandpa "cherish" him..but why oh why he seek his bro's affection so much