What does it mean to *not* be yourself?

Inspired by Being and Time Division One Chapter Four

One of the pieces of advice that seems to make no sense to a young man, especially with regards to dating is "Be Yourself." On one hand, it seems logically strange. If you are not "yourself" sometimes, what are you? On the other hand it also seems practically not very useful, in so far as the person seeking the advice might wish to specifically *not* be…