Relationships can be summed up with some simple Maths in my view. A relationship between A and B is positive and will last if: E(A plus B) > E(A) plus E(B), E(A), E(B). Where E is the expectation of life-time utility defined as as emotional balance, economic success, personal enrichment, etc...

Add to this the phenomena that the more you work at the tools and skills of relationships (my significant other and I just completed a book on constructive arguing for example) then over time E(A+B) should increase, even as the expected years of your life-span and ergo your life together diminish.

It is not about right person or wrong person as if this characteristic was 100% a passive trait of another, which puts all the pressure on partner selection. But about whether the other person has enough of the same life expectations and whether they are willing to invest in a relationship for it to weather the storms of life. When the fire of passion fades to a warm glow this in the end is what will count, I believe.