Emotionally Cancer needs comfort, a feeling of security, a feeling that the relationship is like a downy nest to be in the relationship so that's it's protective.
Protective is a good word because Cancerians are not only protective themselves as nurturers but they also like to feel safe. That's on every level, financially, emotionally and otherwise.
To begin with, if you want to break up with a Cancer, you need to be withholding. To withhold affection, withhold attention, withhold praise and particularly to make the Cancer feel that you're not there for him or her.
It is hard to break up with a Cancer because Cancer's hate loss and hate letting go. They will, unlike many, many signs of the Zodiac; they will hang on through thick and thin hoping that things will improve.
Getting rid of a Cancer is really a long drawn out ordeal because you have to act as if you're cold and you are rejecting, and you are very critical.
In the end, the Cancer will sadly move on but it does mean breaking their hearts. I would say that if you're going to get involved with a Cancer, truly be as clear as you can that you're in it for the long run because to hurt a Cancer really is like stepping on a poor wounded bird.