It seems like Priestly has two settings sometimes, adorable dork and incredibly dark and depressed.
Rambled below since there’s not really a question to answer...
He seems that way because he kind of does have only two major settings emotionally since he doesn’t really get angry all that often, sad/depressed and happy/dorky. Even when he is angry, he’s never been violent, he’s not just a self-proclaimed pacifist, his mind just doesn’t work that way because he doesn’t wanna hurt people the way he’s been hurt. In writing on here, he’s only been truly angry a handful of times and only ever wanted to physically hurt someone once.
With the depressed side, only certain people are ever allowed to see him when he’s breaking. He’s depressed quite a bit, but he doesn’t want to bring people down with him so he’s constantly trying to make people smile. That was a key point in why he started the bakery. He saw the smiles that he caused with some sweets he made for Christmas two years ago and he started looking into it. Sometimes being a dork comes natural to him, other times it doesn’t, but when it doesn’t that’s because he’s having issues with his depression, a chemical imbalance in his head he has no control over.
Not a lot of people have seen him break down completely; Philippe technically hasn’t seen it, though Priestly has called him. If he’s having a breakdown, he doesn’t like people to see it or hear it, but he’s slowly getting used to being able to admit when he needs help. It’s difficult for him to let people see him because his father told him he should just get over it constantly. Al told him to just get over it. He can’t, it’s quite literally physically impossible, but that’s what his brain always tells him is he should be able to just get over it. In his head, he shouldn’t show people that he’s weak so he makes them laugh and ignores that he feels like he’s breaking inside. That comes naturally to him, making it seem like he’s perfectly fine when he aches inside, though that becomes incredibly difficult when your best friend is an empath.