I think we enter in a relationship to accomplish some mutual goals, and to sustain the relationship, those goals need to still be valid.

For example, typically a male and female join to procreate a family and offspring. Ideally this goal would never sunset, because the kids will be still be part of the goals of the parents until death do they finally part. And even the parents care becomes the goal of the kids later on. But the government has so fucked up the nuclear family and the education system, this is often not the outcome any more. It is a tragedy and western civilization will collapse and give way to conservative Asia because of this.

Other possible goals include two very smart people supporting each other's education and creative production, and in my case for example I read that older men live 10 years younger and longer if they marry a younger woman, and I love sports, so I figure the right person would push me and inspire me to stay young.

Framing gratitude and inspirational words in the context of the ongoing mutual goal, keeps both of you focused on the overriding positive of your relationship. Love is the positive reinforcement signal our body is chemically giving us to spur us on towards our evolutionary strategy. If we harness that in mutual goals, then we can sustain it.