Because.. anytime you watch a girly movie and it get's to the part where the guy tells the girl that he likes her, the first thing he says is either, "You're pretty" or, "you're funny."
girls don't like to be thought of as uptight anyways. they wanna show that they can have fun too.
some guys are intimidated by girls who seem very independent or the so called "strong girl" image. so she is trying to tell you that she can have fun too and not as uptight as she might seem to be.
Her motivation is that she realizes that she doesn't have a sense of humor. She wants one because she thinks it is key to overcoming another problems she is more concerned with. She is imitating behavior that she perceived as healthy. Help her to develop her sense of humor, but also talk with her so she will want to figure out and explain what is really bothering her. She views herself as a person that is pro-active about fixing problems. She sounds like a good person with a healthy attitude.