Look, my husband and I don't want to give up either. We are best friends - like two old shoes - you can't have one without the other. But at some point we *may* look at each other suffering, and because we love each other so much, we want the suffering to stop. If I really love someone, than I don't want them to suffer just because I am so selfish I want them to be with me at the cost of them leaving: their family, their home, their career, their sanity. That it is not love, that is ego.
Love is "I want him to be happy - whether it involves me or not." If my husband were to be hit by a bus tomorrow, and die, I would have so much guilt to know that the last year of his life he spent in a clinical depression - barely able to function - because of the stress this process had put on him.
This is without a doubt a cruel and heartless process. It is not for everyone. We should never judge those that give up as having had less love. Maybe they just cared more about their mental health.