a new University of Michigan psychology study found that some people find angry expressions so rewarding that they will readily learn ways to encourage them.

The findings may explain why some people like to tease each other

Perhaps teasers are reinforced by that fleeting 'annoyed look' on someone else's face and therefore will continue to heckle that person to get that look again and again,"

Participants who were high in testosterone relative to other members of their sex learned the sequence that was followed by an angry face better

perceived emotional facial expressions are important signals to help guide human behavior, even if people are not aware that they do so.