Her character's main gimmick is wanting to be in a relationship/get married but her drill sergeant personality when she's on the job pushes a lot of people away.
to be precise she couldn't help but worry about her subordinates if she doesn't got full drill sergeant, so another way to look at it the reason she is single is the fact that she cares so much for her subordinates.
also her fe, valentine and white day's event is the sweetest among all characters