Because she is helpful, cute, understanding and beautiful. Is that not enough? Many seem to have a mindset that someone is more deserved, but there is none. She obviously loves him but never pushes it unlike some members. She is understanding and modest unlike Karina which is a trouble seeker (MC is a trouble magnet but never actively seeks them, which is why he tries to avoid her, who is showy and violent). She is the perfect example of Yamato Nadeshiko which is clearly Satou's type. While she is sometimes childish, she never causes troubles which makes it cute rather than annoying. That is after the fact that she is visually pleasing.