There are two reasons. 1) It usually gets worse when he's excited or nervous. Selfishly, I can take this as a compliment for me 2) It is a vulnerability of his. He's a strong person who has a problem that other people can see. His being able to deal with makes him even more attractive. BUT u/suavro mentioned that a stutter can be something that 'goes deep and shapes a lot of a person's personality' which is something I hadn't thought about before. So I will make sure I really understand his own attitude towards his stutter before I say that it's cute.