I think a huge part is how we're socialized growing up to see our value and worth as being tied into a relationship and how our culture teaches us a distorted sense of romantic love - can't live without you, can't breathe without you, I'll die without you. As teenage girls we believe that level of emotional intensity and dramatics equates with real love. We're also taught that if we date lots of people, then we're sluts, so at an early age we put all our eggs into one basket, so to speak, and concentrate on "the one".