Because only men point of view is considered. The premise is that your happiness deride of making a man happy. That is a man is happy, then he would treat you better and then everything works out. At no point they consider that making a men happy might make you unhappy or be indiferent for you. They have believed the movies that once you give them a chance, your life would be filled of "cute" moments movie worth it that would make you fall for him a prioritize his happiness above all. Otherwise they might have to start considering that their brother is making other women uncomfortable or maybe that the coworker they get along is a creep with women or... And that is quite unconvinient when you do benefit from the relationship with that man.