Well, however difficult it may be to hear, love can just fade away.
And it never comes out of the blue. Someone doesn't stop loving you from one day to the next. Meaning the one who didn't see it coming didn't pay attention to how the other was feeling and reacting.
You should never take a relationship for granted, as a kind of status quo. Relationships are living entities.

As for appreciating the partner being loving and giving... Has it dawned on you that you can smother someone to death?
My ex husband did whatever to please me and boy does that wear thin! I couldn't even say, "Gosh, I'm hungry!" or he was in the kitchen making me some food.
That attitude began to annoy me, then irritate me, more and more and made me lose respect and with that my love for him.
People want a partner, an equal, not a servant, not a pleaser either.
You know the saying "if it's Christmas every day it isn't Christmas anymore"?
Doing something great and romantic regularly is cool. Doing it very often isn't.