when they learn that they can make a mess without doing any lasting damage, and the behaviour is rewarded by them getting what they want (in this case, the girlfriend stopping saying "I'm leaving you" and reverting to Happy Relationship). It's along the same lines as toddlers throwing themselves on the floor and beating the ground until their fists turn red and screaming until they make themselves puke - it's not nice, in fact it hurts, but it makes mummy/daddy run round and fuss over you and worry about you and clean you up and Make It All Better.