Thinking thinking thinking

About all sorts of things, but mostly about the nature of relationships and work.
There's this theory that one needs to work hard at keeping relationships going and heading in the right direction, along the same track. That being in any type of relationship is hard work, because it's about individuals melding their desires into one cohesive sort of whole.

But I'm beginning to question the wisdom of such thinking. Is it sensible and sane to throw your heart and soul constantly into relationships that simply do not improve, that don't seem to get any better? It's exhausting work, and there seems to be no return on it.

I understand that relationships are a trade off between what you believe the final result will be worth, compared to the effort that you are expending to get there. Or am I being far too 'capitalist' with my thoughts? Am I treating a relationship experience with the jaded attitude of a business person? Trading profit versus loss in an emotional sense?

And if I am - is that wrong? Surely the point of life is happiness, and the point of relationships is to share that happiness with people in a way that engenders MORE happiness?

Challenges arise on every path, but if there are nothing BUT challenges, is that path worth pursuing?

Much thinking to do here. And probably much more rambling as I think. You might have to bear with me on this one.