Tkeybel, don't be thrown by the passion thing. I get told she doesn't find me attractive, I'm not fun and she's not sure if she still wants to be a couple with me. If that were true, they'd run and not look back and not be caught in this limbo mess.
Passion, sex, lust, attraction and romance are all just drugs they use to create a wall between them and their issues. I read through W's diary when I was cleaning out her stuff and I can tell I am the only person she has truly loved or wanted to be with. Every other person, she was clearly in love with being in love. When you love someone unconditionally and as an equal, that other person is left as another part of the whole and in being such, they are left to deal with themselves as individuals, often for the first time in their lives.
Kids, marriage and careers often provide just enough of a barrier to keep them separate and then one day it's gone and they don't want to look inwards. They then find someone else as a drug and use it as an excuse to justify never having should have been with you in the first place, since you leave them so dry and empty feeling. When all it really is that they wanted to avoid having to look long and hard into the mirror.