If reciprocity and gregarious behavior are basic to social creatures, then exclusivity and rejection would seem to be aberrant. But, what is the cause?

Does an antagonistic stance prompt an automatic ameliorative response, based on the perception that perhaps gregarious intent and outreach was simply misunderstood and so the antagonism or rejection is inadvertently rewarded?

Fact is that a giver needs a taker; a taker does not need a giver. The taker can just, if he is a human, steal. There is no thievery in species that do not recognize ownership.

It is commonly understood that some children become suspicious of new experiences or persons. It does not happen in all. Why some? What triggers the impulse to exclude? Is it simply normal in some children or is there a precipitating event?