He honestly only did it because STUFF was going on and he just got back from being lost in the caves by himself and felt like hanging around a lot of people, even if they were being traumatized and such.

He didn't push Nora away because he figured there wasn't really any harm in letting him cling and stuff and he knows what it's like to be going through something super crappy and not really having anyone to turn to. He also figured it actually didn't matter to Nora who it was, so if Nora starts annoying him later it wouldn't hurt anything if he decided not to be friendly anymore (this is a stupid plan on Katou's part because he's actually kinda clingy himself so he's probably not that great at rejecting people that like him).