As a middle-aged person currently going through a divorce let me say that there are many reasons…
Aaron Brand

Respect and trust.

Willingness: to grow together, and to be able to give each other some distance.

If a marriage is a partnership for life, then things should be less focused on Love/Lust. Love/Lust fades. Love that is built up over time through respect and trust will never stagnate. It will only grow.

I have been madly, crazy in Love/Lust — where I genuinely believed this person was The One — it didn’t last. We didn’t work out. Who we were, were simply too fundamentally different from the other. In a way that clashed. At the end of the day, we just weren’t willing to put in the time or the effort to try to understand where the other was coming from, or what they needed.

Conversely, the best relationship I ever entered into with someone was an experiment in partnership.

We entered into it knowing that we were both broken over other people. There was never any lie involved that we were anything more than just comfortable. 
We were friends first, and then we decided to give it a try. 
There was no facade and no maybes involved. (I mean, we had a relationship agreement, ala Sheldon Cooper & Amy Farrafowler. Hehe.)

We went in knowing that we were not in love with each other, and that we were only just passably attracted to each other. What we gave each other was space when necessary. Conversations that genuinely acknowledged the other’s thoughts and emotions. This translated into actions that were governed by trust and respect. Instead of restricting each other, we were able to grow — together, and then finally, apart.

I still hold an infinite amount of respect for him, and I still trust him implicitly. Which is something that I can’t say about any of my exes whom I was in Love/Lust with — not because of anything, but just simply because I didn’t really want to know them beyond a certain point.

At the end of the day, these are just my personal beliefs. I won’t go so far as to say that I’m deeply jaded, but I am a child of divorce. As a child of divorce, marriage to me is extremely sacred. I’d rather not get married and be with one person forever, than to get married and divorced just because I fell out of Love with someone, or simply because neither party wants to work on it any longer.

I don’t know your story, so I hope you understand that this is in no way a personal attack, or carrying any judgey undertones. This is simply my perspective and my experiences.