Anyone got any theory about Itsu's secret/past?? About Kousei something?

I'm thinking that something happened on his childhood during his flight back to Japan? That changed his perspective and want to travel to teach children(?) Because that's what he's been hiding from yuki and his goal in life, but yuki's father alr know about his past and idk they look solemn when they talked at the hot springs, so I guess something tragic happened on the flight or idk but the way oushi hides it, something def happened, as he said before that Yuki is pure and has never been tainted(?) Was that the right word, I just remembered that he said around ch. 3 or 4 something, idk if that has any connection with kosei unless he was on that same flight