Ok, listen, LISTEN
He Tian is a really complex character. He's been trying to escape the darkness that his family is involved in his whole life. But at the same time, he very clearly thinks he inherently has that "evilness" inside of him. (it's the simbolysim behind his "fear of the dark" and the reason he lists Mo not being afraid of the dark as one of the reasons he (ht) likes him)
And then, THEN, Mo guan shan shows up, and not only he sees that darkness and doesn't back down in front of it, but he also starts to care for Ht despite it being there.
So the thing that starts growing inside He Tian is the fear that this one person, this one boy who doesn't pull him deeper into the darkness (like his family (sorry He cheng i love you ) bringing him into the business) and KNOWS him as more then just that, will forget him.
That this side of him, that can be good and loved without ties of debt, will disappear alongside Mo's memory.
Then She Li shows up. He tian sees with his own eyes that he cannot protect his loved one without that darkness, and above all he can't freely protect him if that darkness isn't entirely his. This time when She Li attacked Mo he had to bargain for power with his brother to take SL down, so he faces the fact that he needs power and influence to keep Mo safe.
So there comes the sacrifice, he'll go away if he can save Mo. And while away, he'll conquer whatever he needs to make sure Mo never struggles again, so he will never need to leave again.
BUT here's the thing. Mo doesn't know all that, he only knows what HT has told him, Mo knows He Tian only ever expressed fear when 1 - Mo was in danger, 2 - When faced with the idea of being forgotten.
And after months of being loved, reassured, and having HT promise to give him the future of his dreams, months of letting He Tian in, he just disappears one day.
And suddenly Mo is in the position of being "left behind", forgotten.
So when HT comes back, this smug fucker who hurt him that bad, acting like nothing ever happened, Mo wants to pretend it doesn't matter. That he wasn't "the dumbass who got left behind", that it never hurt enough to still matter, years later.
And he also wants it to hurt He Tian back. So he says this.
He says it knowing it will hurt. If he could be left behind by Ht, he wanted he tian to be hurt by the fact that Mo could also forget him. And it does hurt, you can see it in HT's face, in the fact he has to turn it away.
But Mo does care for He Tian, we all know this. So as soon as he sees the result, Ht's hurt expression, the words "you're so cruel", as soon as he sees it actually hurt, he gets hurt as well. He feels the conflict that this situation would bring, the feeling of resenting someone you love, or/and hurting someone you love.
And finally, He tian can't stand it, he can't leave, can't let himself be forgotten by this precious person. So he goes back, he insists on "if you don't remember me ill stay until you do". It's a desperate need, a deep fear.
OX has always told their story in very subtle ways, the plot is always very spread out, and you have to pay attention to the words and details they put in the story. It's as frustrating as it is fun to read it lol, he tian going back and seeing Mo naked is a fun joke, but I do also think it's something that keeps his characterization and does somewhat progress the plot.
TLDR: Mo pushed ht away because he wanted to hurt him for leaving (and he knew how to really hurt him), but also got hurt/felt guilty for hurting someone he loves. Ht came back during the shower because he's scared to let Mo forget about him.
