Unfortunately this is very common not only in relationships/marriage but as a trait of human nature. Most breakups are the result of someone changing. "We treat the new better than the tried and true."
Once people get to a point where they feel they can RELAX they tend to slack off and put in less effort.
After being out of work a person gets a new job. During the probation period they show up everyday on time and never took a sick day off. Five years later they come in one day 20 minutes late and a co-worker teases them. Their reply is: "They're LUCKY I showed up!"
A person gets a new car and tells their friends no smoking, eating, or drinking in my car! They have it washed/waxed each weekend and detailed every 6 months. Three years later it's got an inch of dirt on the body and the inside looks like a grenade went off in it.
During the early courtship a man shows up at his girlfriend's house and she asks him if he would like something to eat. Four years later he comes by and asks: "Baby, will you make me a grilled ham and cheese sandwich?" Her reply: "You got two hands fix it yourself!" or "I worked all day just like you! You know where the refrigerator is....etc"
When a couple first gets together they make each other's happiness a top priority. They bend over backwards to impress and please one another! His focus is on (her) and her focus is on (him).
The word "no" is seldom if ever used. The longer a couple has been together the more incline they are to say "no" to a variety of things.
They lull themselves into believing "true love" means they get to STOP doing all the things that won your heart and you won't walk out on them. Taking people and things for granted is human nature.
A couple really has to make an effort not to fall into that "comfort trap". When we change our circumstances change.
It's easier to maintain a fire than it is to reignite a spark!