Human beings know who are good to them, and thus they know they can be nice to these people as well.
Human beings also know who don't like them or who backstab them, and thus they dislike these people.
Human beings who like these group of people will do as much as they can for them, to care for them as well.
Human beings who don't like these group of people will either tend to ignore and avoid these people, or backstab them back.

It's that simple.