Skinner Box

Grinding is basically an underhanded way to try and get you addicted when implemented to the degree you'll find in MMO's or some of the later FF's where grinding is more and more prevalent and obvious.

That said, I beat every FF game ever made without resorting to item grinding and generally refuse to participate in such exercises simply because I feel it's a cheap and easy way to try and addict people without having to spend the time actually designing gameplay that is inherently enjoyable.

This is also the reason I don't play pretty much every game Zynga has ever made.