It’s that route, you know? Like - his first time seeing someone not fawn over him... He’s finding hard-to-approach attitude attractive. He probably thinks she’s being a tsundere and it’s a new seduction technique. And assuming this all-on-his own, he thinks of this as flirting and is angry she’s cheating on him , thinking about someone else while flirting with him. That would irk him & make him mad.
But, it being the result of his own false presumptions, just makes him look like a fool, to me it’s funny, actually 🤣🤣🤣