You didn't knew what it was specifically about Katsuki, but despite how much he yelled and threw insults left and right, there was no denying he made people around him feel safe, one way or another.

He scowled, but he never strayed away from someone that he felt needed protection or support in any way.

He yelled and told everyone to fuck off, but he was observant, and he knew what was going around him all the time.

So it didn't took him long to notice something was wrong with you.

However, he knew that pressing the matter could make it worse, and so, he let you come to him when you felt ready, regardless of his own personal desire to force the truth out of you by force.