What's the word for what I'm thinking?

I know I learned it in Economics senior year and it was the last semester, I had Senioritis, but I remember the example she used.

When you buy a big mac and you really love it, you keep buying it and eating it but eventually the more you keep buying it and eating it, you love for it will decrease.

What's the word or phrase? It's killing me that can't remember it and I need to use it in a story I'm writing.
 
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