I find it to be the opposite much of the time. Most people fall "in love" (suddenly) and fall out of love (gradually).
Usually what happens is their mate slowly STOPS doing all the things that caused one to fall in love with them. Romance & passion dies.
(It's easier to maintain a fire than it is to reignite a spark!)
"We treat the (new) better than the tried and true." In the beginning both people made it a (priority) to add joy and happiness to each other's life. Oftentimes once people get married they scratch that off of their "things to do list". It's not uncommon to hear someone say: "He/She is NOT the same person I fell in love with." When we change our circumstances change.
Another possibility is sometimes people evolve in different directions. There is no neutral in relationships. You're either "growing together" or "growing apart". If one is not careful it's very easy to (sleepwalk) through a marriage/relationship. One person assumes that is the way things are supposed to be and the other person quietly longs for the way things use to be. Words may hurt but silence kills.
"There is no amount of (work) or (communication) that can overcome being with someone who does not want what you want."
There are only two ways to experience joy and peace of mind in relationships: we either get what we want or we learn to be happy with what we have.
Last but not least sometimes people wake up and feel like they "settled" not just for the person they're with but in life in general. Some people call it a "midlife crisis" others call it a "midlife awakening". Either way it's the recognition that one has more years behind them than they do ahead of them and they want to make the (most) of those remaining years. It's not unheard of to hear about people quitting established jobs, ending marriages, or relocating in order to pursue dreams they gave up on.