I hate it when people crack their fingers. The snapping noise just brings up my hackles; it sounds painful, and then I think about someone breaking my fingers, and then I wince and want to stuff my hands in my pockets to protect them from vengeful finger breakers. It is the most aggravating thing a person can do to me.
Almost everyone has that sort of thing--the one that they cannot stand. Maybe it is a twitch every time they hear the word "darling", or a certain level of discomfort with people coughing. Regardless, we usually don't put those little humanizing things in books. We may mention big weaknesses, but not the little ones.
So here is your task: give your protagonist something they hate. But--this is key--don't tell the reader. Just keep on showing that they hate cracking fingers or loud burps. Readers are smart; they can pick up on the quirks. Don't be explicit. Show, don't tell, your character's hate.