In this thread, someone suggested that we take the titular topic to another thread, so here it is.
Not interested in passing judgement here, tho I do wonder how things can go bad, or at least to mediocre. I guess people just grow apart more often than not, energies get out of synch (parse that how you will), trust may be abused or shattered, etc.
Even more mystified how you can marry when there isn’t a spark there in the first place, or there aren’t any shared interests or passions; just unfathomable to me how people can tie the knot in such situations, but perhaps extenuating circumstances abound.
Of course this OP is coming from a lifelong bachelor who simply has resigned himself to pursuing his adventures alone (since more often than not I feel like I am dragging an anchor along with me to some wilderness spot, and not a fellow sojourner), so it is quite possible that I am missing something. Hell while I’m at it I’ll start yet another topic on that…
I think about this a lot (probably way too much.) Sounds cliche, but I truly believe the crux of it comes down to a lack of communication… if “energies get out of sync,” it’s not discussed. People start to assume things, often negative things, and those things have a tendency to build upon themselves until bad things happen. Often so much bad stuff has built up that people convince themselves that it’s simply easier to start over than to try and fix what they have.
I’m sure there’s a balance here, because over-communication can become tiresome too. But I think people should talk more and assume less in general 
Lack of communication is a huge part of it. Little misunderstandings can grow huge over time. We all change as we go through life’s phases. Married people can change in different directions as they go through the years, especially if they don’t communicate well.
The values and interests my wife and I had when we were married 25 years ago are barely recognizable to us now. We were young then , now we are old. We were childless and free then, now we have adult children expecting support. We both rented apartments and worked temp jobs, now we have a house and a dog and permanent full time jobs. Never one of us look like we did back then, that’s for sure. We really aren’t the same people anymore and sometimes I wonder how we got here.
So, it wouldn’t be that remarkable if we grew apart, except that I married a person I like, a friend. So we communicate, sometimes with a little help, and that helps us find common ground. At the end of he day though, I think it is respect that keeps a marriage together. Respect allows my wife to give me the benefit of the doubt whenever a problem arrises, rather than assuming the worst, and respect allows me to do the same.
My marriage ended by me because he wanted a lady to be his arm candy. Eventually arm candy wasn’t going to help him with the children or be at his side when things took a bad turn. Arm candy won’t cut grass or buy food on a budget. She won’t help care for ailing parents …he choose pretty over love and devotion …now… he can sleep in that nasty bed he made…
We are growing apart because, while we both want the same things, I am the only one working towards them.
Just because I can do 99% of the work required to keep our house, family and kids on the up and up, doesn’t mean I will keep doing it forever while he (unintentionally) sabotages our lives.
Eventually, it adds up and you start to resent the other person.