I should do a headcanon post on why Shunsui is scared of being in a relationship.
Well, you know what, here we go.
Growing up, Shunsui had always been known as the lazy, young man. Who Personally, didn't want to even be a Soul Reaper. He didn't know what he ever wanted. His family didn't support anything he did until he started to go to school.
He wasn't close to his brother, or his parents. They were just kind of there. He didn't feel loved, or cared for, since most of the time he did it for himself, or got it elsewhere. Even so, that elsewhere didn't last long for him Growing up, getting stronger, when he noticed his brother's reaction to his wife, made him think about how comfortable it would be. He was allowed to get closer to his brother, and his wife.
Everyone he started to care about, started to pass away, and it was hurting him. Thinking it might have been his fault or the curse's fault, he after many long years told himself he couldn't fall for someone. Watching those being ripped apart from death. ( Seeing how his sister-in-law was broken after his brother died.) he didn't want that for himself. He wanted to be stronger, for himself, Not allowing others to get close enough to him. Building his walls. Yet, he might be goofy and happy, but that is all a front. Even his weapon would kill the person he falls for if they got in the middle of a game with them. ( she is very jealous of Shunsui being close to anyone.) Katen Kyokottsu was jealous because all she wanted was for him to play the Ulitmint game with him. ( After all, to death or us part with those who are the closest...)
Anyway, thinking about how everyone he loved would die. Could be used to harm him, harm his soul, or get in his way. He never wanted to feel empty inside. Loving someone for years, watching them drift away, as he stays back in the shadows.
Shunsui just didn't want to feel empty watching anyone he loved leave him.
So he thought if he didn't get close enough to people, he would feel better, but watching those leave him, made him realize that his feelings wouldn't matter, he can't stop what is going to happen.
He was always going to be alone.
The walls he has built would be the hardest to break down.