If you have not yet interacted with a person, you are judged neutrally by them. If you do something for them once, then you move up in their eyes. If you continue to benefit them you can move further up. If you stop you move to well below zero; you have actually slighted them. Even if you slow down a bit you can go into negative territory. This goes for many things humans offer each other from tea to sex. Why is limited attention worse than none?
One guess is that it’s an upshot of tit-for-tat. If I am nice to someone, they are nice to me in return, as obliged. Then I am obliged. Mentioning that the interaction has occurred an even number of times doesn’t get you off the hook; you always owe more friendly deeds.
Another potential reason is that when you haven’t interacted with someone they still have high hopes you will be a good person to know, whereas when you know them and cease to give them attention, you are demonstrably not. This doesn’t seem right, as strangers usually remain strangers, and people who have had an interest often return to it.
Perhaps un-friendliness is a punishment to encourage your future cooperation? People who have been useful in the past are a better target than others because they are presumably already close to being friendly again. If I’m wondering whether to phone you or not and I think you will be miffed if I haven’t it may push me over the line, whereas if we haven’t met and I think you might be miffed when we eventually do, I probably won’t bother because I probably will never meet you or want to anyway.
For whatever reason, this must reduce the occurrence of friendly behavior enourmously. Before you interact with someone you must ascertain that they are likely enough to be good enough for long enough that it’s worth the cost of their badmouthing or teary appeals to stay if you ever decide they’re not. This certainly limits my own friendliness – often I wouldn’t mind being helpful to strangers, but I’ve learned the annoying way how easy it is to become an obligated ‘friend’ just because you can’t bear to watch someone suffer on a single occasion. So other people prevent me from benefiting them with their implicit threat of obligation.
Interestingly, one situation where humans are nice to one another and not further obliged is when they trade fairly at the outset, such as in shops. This supports the tit-for-tat theory.