you accidentally left your diary open on your desk while you leave your dorm room to get something, and Katsuki decided to look through it as a joke and expose your embarrassing secrets like crushes or flops but then...he opened it and immedietly, what was written on it was eveything you've vented in the diary which was trauma and stuff

"..." he gently placed the diary back, the way it was. Also learnt all the stuff you never really told about, since you're not really open to any of your friends. Which, was all personal that was not meant for anyone to touch. "Yeah, I'm just gonna pretend I didnt read all that-" he muttered to himself, not wanting to seem to be snooping in anyone's personal belongings. So he'll just leave it there, and act normal before you come back.