From the time they are babies, girls absorb hundreds of micromessages each day telling them that they should be nice, polite, and polished. Adoring parents and caretakers dress them impeccably in color-coordinated outfits (with matching bows) and tell them how pretty they look. They are praised mightily for being A students and for being helpful, polite, and accomodating and are chided (however lovingly) for being messy, assertive, or loud.
Well-meaning parents and educators guide girls toward activities and endeavors they are good at so they can shine, and steer them away from ones they might find frustrationg, or worse, at which they could fail. It's understandable because we see girls as vulnerable and fragile, we instinctively want to protect them from harm and judgement.
Our boys, on the other hand, are given freedom to roam, explore, get dirty, fall down, and yes, fail - all in the name of teaching them to "man up" as early as possible. Even now, for all our social progress, people get a little uncomfortable if a boy is too hesitant, cautious, or vulnerable - let alone sheds a tear.