Watching the Couples Go By
Third, the woman serves the man's need to be needed. If no one needs you, what good are you, and what are you here for? Other people--employers, students, readers--may say that they need you. But it isn't true. In all such relationships you are replaceable at some price. But to this woman you are not replaceable at any price. And that gives you the self-esteem to go out and meet the world every day.

http://slate.msn.com/id/2562