So maybe it is part emotion but probably more choice. It is easy to stay and Love when things are going well and positive emotions are being experienced. Much harder to stay and Love when things are not going well and things are challenging.
I also don't think there is a perfect partner out there for any of us so if you do choose to Love you are willing to accept that person as is and all of their flaws. If you're not open to Love then I assume you will dismiss the person when you observe their slightest imperfection/flaw.
The Dr. has said that she will get married again but when we are out she comments on all the married people that are on their phones, seemingly disconnected from what is going on with their partner. I am definitely guilty of that when I was married.
I guess in order for it to last it takes two people that are very aware of what it takes to make Love and a R last, that are willing to communicate about it and continue to share the same values about commitment, etc?