These behaviors are enforced by ourselves and others. Gender differentiation is a process that begins at birth. Female infants are dressed in pink and their rooms are decorated in pastels. Parents give subtle messages regarding gender and what are acceptable behavior for each gender. As children move into the larger world of friends and school many of the ideas and beliefs are reinforced by those around them.
Conformity is a major factor in human behavior.
This notion adds support to maintaining the idea of gender differentiation. Traits associated with males tend to be more highly regarded than traits associated with females. This may help to explain why it is easier for females to cross-over the gender stereotype barriers and harder for males to do so. We would not even consider a female dressed in pants or jeans unusual but a man in a dress is still considered odd. This works for color too, females can wear blue but males have a harder time wearing pink.