>Then we could have an honest conversation about why.

It doesn't even need to be a conversation. Humans forget things.

"Oh damn, I forgot! Let's grab dinner tonight!" or w/e, that's all people need.

I don't disagree, but sometimes there could be something else going on.

In my marriage i have been slowly trying to move my birthday back one day at a time. After 3 years of effort ive successfully moved it back 1 day and my spouse still gets confused if its 1 or 2 days later, all due to my active efforts. I dont believe anyone should do this in a marriage, but they could!