restrictions for only girls | feelings and emotions

When I was a kid... my grand mother used to say me that... ''you are a girl, do not laugh so much''
I am older Now I understand that why she used to say me like that....
Too much laughter becomes a burden for girl. Because if a girl laughs too much people thinks she doesn't know about manners... or she is characterless etc....

Why... I don't understand... why these restrictions are only for girls...?