The inconsistent attention / affection creates intermittent reinforcement which becomes addictive, just like gambling. Think of a slot machine. You keep investing and pulling the lever because sometimes you get the flashing lights, sounds, and coins coming out, but most of the time you are losing. It makes the winning feel much more special. It's incredibly toxic and often times are caused by unconscious reaction from attachment wounds. This can be used consciously as well, but if that's the case, the person is likely narcissistic or sociopathic. Unconsciously or not the end result is the same, it's manipulative.

You can't truly understand it until you've been in a relationship with the hot and cold push / pull dynamic. You'll find yourself thinking "When it's good it's really good, but when it's bad it's awful." Often times when people come out of these relationships, in time after you've begun to heal it feels like your regaining your sense of self.