How do you get into a position where you have to "dump" someone?

I was asking myself if I had ever dumped anyone in my life, and while I have been the one to initiate the end of a relationship, I can’t think of a time I have dumped anyone. This, however, could easily be because of my bad memory. I tend to forget bad things pretty easily, and these days, I have a hard time remembering what questions I answered on fluther an hour ago.

Still, it seems to me you have to arrive at a position where your expectations from each other are quite different if someone is going to have to do some dumping. That means that, on some level someone’s expectations have been raised past where they should have been.

How does that happen? How do you get to the point where you have to dump someone?