Surprisingly I think a lot of people "let themselves go" because their partner is already neglecting them! They feel like "what's the use."
Others look at finding a mate as something on their “things to do list”. Once they have found someone to “settle down” with they RELAX. I've heard women in particular exclaim, "Whew, I'm so glad to be out of that dating scene. Once you're married you don't have to worry so much about how you look or act." There is an old saying that goes
Many of them STOP doing all of things that attracted their mate into their life and yet they expect this person to maintain the same amount of love and attraction towards them.
(Oddly enough if the relationship/marriage came to an end they’d be back in the gym!)
We’d all do better to remember (the same thing it took to get your baby is the same thing it’s going to take to keep them)
“When We Change, Our Circumstances Change!”