I understand you, I often feel that way. :/
(But I still think this is a little cute )
I can see where you get this point of view;
however I think that it is supposed to portray his child-like qualities through his juvenile actions.
Thus giving you a sense that he is younger than the others.
So not only should he be un-adult like but this attribute also aids to demonstrate that he is in the early stages of loving another.
As well as this, you say you never see people acting this way but, surely characters should be an exaggeration of realistic behaviour or it would be dull, no?
╮(￣▽￣)╭
I agree that an exaggeration of a character can make a story better but in this case it doesn't work for me. Early stages of love with his big headed and almighty behavior doesn't make me like the main character. In order for me to enjoy a story I have to in some way understand, like, or interested in his/ her actions. In no way shape or form am I interested in his life because I see NO likeable qualities. If you like the main character then you enjoy these type of stories.
I feel I would like this monga more if the main character was such a cowardly bully. He acts all tough and then puts on a stupid I'm scared/ into you type of expression. He gets super jealous and act all high and mighty but is really childish with behavior and how he handles real life situations. ( never seen peolpe act this way when liking someone) I need my characters to be adults about their feeling or at least talk honestly about them or it frustrates me as a reader （￣へ￣）