So I read the Novel, and it was a bit clearer to understand than here.
Basically Jooin hides his true self (personality and intelligence) in fear of people not liking him if he is t super bubbly and happy. He thinks this makes him a bad person. Dani reassures him that the fact that he doesn't want people to be hurt by being disappointed in him shows he's a good person. But even so, even if he wasn't 100 happy and bubbly his friends would still like him and stay by him.
He also worried that Dani wouldn't understand him because she just forgave everyone in the classroom who gossipped so she's like a saint. And how could a saint understand someone bad like him? But Dani said rather than just forgive them, it really is she doesn't care to give the time and energy to worth about that class because she has such good friends and is happy.
ooh thanks for clearing it out for me! :D
Chapter 112 I didnt understand shit I'll just pretend that something sweet happened