They're too insecure to depend on themselves. Girls think that having a boyfriend is a sign of desirability. They think if they don't have a boyfriend other girls will think there is something wrong with them. Do you see a trend here? It's all about what other people are thinking.
I would depend on my boy friend because, I can really trust him, and so I won't feel so alone but that's what I have my girl friend for, but it would be cool to have a boy friend so I can give Kaity a break
Co-dependency issues. Some people are afraid to be independent so they depend on others instead. If it's not a significant other than it's someone else, like a grown child in the case of my mother in law.