In long term physical relationships, I can't help but to think that sometimes one of the things that is being exchanged are psychological tools and skills for dealing with life. The longer one spends with another person, the more of their life skills we often pick up. The only problem with this (if it is a problem) is that we often don't have an unlimited reservoir of these skill to teach. Once a partner has learned all we have to teach, and often we are very choosy on the things we can or care to learn. this is one less thing that can go into the equation of the exchange in a relationship. That popular guy or gal in a bar may be popular because they have unique or effective ways of looking at things. They may come across as funny or charming - intellectually stimulating. 3 years or so down the road, that guy or gal may not seem as funny or as charming - maybe this is one of the reasons why? This may contribute to an imballance in the relationship.
Even optimism is a skill for dealing with life, and many people find creative optimism attractive.

Intersting. I like it.

that is perhaps the reason for many intersexual break-ups; the fade to black of whatever the hell you saw in that fucker in the first place. The always awkward and inevitable confrontations of reality.

always awkward and inevitable confrontations of reality

I thought this was rather insightful - in the way a realist might think - I mean how many relationships has one had these days on average when a person turns 25?. And just off the top of my head - It makes me think that maybe what we regard as love and commitment is sometimes expressed as a promise to struggle against harsh realities. And perhaps when we regard this struggle as richer for facing it together - (I think some relationships are actually stronger having had adversity) that may be the strongest type of relationship. Of course there are many relationships that have few harsh realities - and the forces keeping people together is much stronger the the forces seperating people. This is one of the good thing I would say about religion is that often it is a strong force keeping people together. People get support in church.