:)
i belong to you is a way of saying i love you. what is love if not a place of belonging?
in short it is a means of validation, a point of saying "i will follow you, i will be yours, i will exist for you, and save myself for you" this is the epitome of control, there is no deeper level of submission than to that which you crave and desire so much that you willingly give yourself up for it...
it is the devils contract... a submission of soul (and will) for the object of your desire.
Your Fiancee may very well be a sociopath, that doesn't make him a bad person. Regardless of what some would like you to think, it simply means he views the world through a different lens. If you love who he is, and he wants to give himself to you, then who's to say that isn't love?
If you try to take control of what opportunity you allow yourself to engage with, you will usually destroy its original form and therefore change the outcome, beyond what would be the original purpose of the intended object.
Cause and effect.
The trick is to be able to understand the approximation of the effect, before the cause is engaged.