There are pretty much two schools of thought.
The first one is some people actually expect their mate to continue being the person who won their heart. They hate "bait & switch" traits.
The second one is a lot of people actually EXPECT to be able to "relax" or "slack off" once there is a commitment or "emotional investment". They consider it to be "normal" to change priorities.
For them it's like completing the "probation period' for a new job. During that time they always came into work on time and never took a sick day. Once they felt the job was "permanent" they stopped putting in the same effort.
Generally speaking no one ever tells their mate when they meet:
"I hope you know that in a year or two from now I'm not going to be buying you flowers, cards, gifts, planning surprise weekend getaways "just because". You can forget about spontaneous passionate sex, wet kisses, and snuggling on the sofa watching TV or talking at length about our future together. Once I know you're (mine) I'm going to STOP doing all of that crap!"
And yet people who do such things expect their mate to stay with them. What other scenario can you imagine where if you STOP doing something everything remains the same?
When we change our circumstances change.
It's almost a cliché to hear someone say:
"He/she is not the same person I fell in love with."
Stop going to work you lose your job, stop paying your rent you get evicted, stop paying your car note it gets repossessed, stop nurturing a garden it dies. It's thin line between being "comfortable" being taken for granted.
Such behavior simply makes it all the easier for a stranger to come along and make him/her feel "special" and "alive" again.
It's easier to maintain a fire than it is to reignite a spark!
“While we are free to choose our actions, we are not free to choose the consequences of our actions” – Stephen R. Covey