Hiroki can be fairly self-absorbed, but he's not actually very emotionally stupid. So he can see there's quite a bit that Chitose isn't talking about, and the happy mask irritates him. But he also knows that to hassle Chitose too much would be hypocritical and annoying! And, well, he understands missing somebody, and he's uncomfortably aware of what a wreck he'd be if Nowaki had come and then gone, especially earlier in their relationship. So the little he picked up about the Koganei thing... It makes sense to him.

Other than that niggling thought, Hiro thinks that Chitose is a genuinely nice person and relatively unlikely to tease or fluster him. Which makes him a good person to talk to! He considers them somewhere between acquaintances and casual friends.