What Makes Relationships Sustainable?

I was in conversation with a friend of mine. We were talking about some other group of people when she stated that "my relationship with them is now normal becasue they do not have a stake involved with respect to me". Now this made me ponder and I did not like what conclusion my mind was trying to draw :|

Are relationships 'interests' based? Is all that talk of love, compassion nothing but just another word of "protecting ones interest'? Every relationship starts based on the feeling of love and care. I agree to that otherwise no relationships would develope. Yes you can argue that not every relationship initiates on the feeling of love e.g. two business partners who although have a relationship but not out of love but to 'further their interests'. And we cant really put sustainability factor in this situation. What I am talking about are the relationships built on the foundation of love. Father and son, mother and daughter, siblings, husband and wife(?), lovers...

Consider for example a son and a father relationship. Pure love eh? Right. Everything is going ok between them. A dispute arose between the two, suppose, a property dispute. The father and son dont see eye to eye on this one and in the end the son is thrown out of the home. (yeah quite exaggerated and quick but this happens).

You can say that since a 'negative' element was introduced in the relationship of love so it ruined everything... but can it be said that 'threat level' of negativity rose to such an extent that it prompted the father to 'protect' his interests? Was the underlying feeling between the two of 'threat maintenance' to a possible maximum? Is love just a cover up?

Ok how about saying that a relationship consists of both elements - 'love' and 'threat maintenance'. Does the latter outweigh the former? Countless incidents of love and sharing are equalled by just one negative act e.g. being unfaithful.

What makes relationships sustainable? Love and compassion for the other OR maintaining that negativity threat to a lower level? Is the glass half filled or half empty? Are they one and the same thing?

Am I nuts?! What the hell am I saying?! :p