Joseph asks, clearly frustrated. The question confuses Haneul. Of course he wants to be happy. Anyone wants to be happy. He never denied himself that, ever. He just never has the opportunity.
Joseph probably said that because he doesn't understand how hard it is to wake up every morning just hoping nothing triggers his trauma or mental issues and making it to bedtime only to pray the nightmares will leave him alone. Every single waking time is a struggle to stay sane. He has no time to think about chasing his happiness. "You don't understand. It's not as eas-."
"I just-," Joseph cuts him off, now his voice kind, "think you deserve it. You paid the price, it's time to get your reward, don't you think?"
