In a study where subjects were shown pictures of people they don't know, then asked to guess whether or not that person may or may not reject them, the heart rate of subjects decreased when rejected, and decreased even more when unexpectedly rejected.. that is.. when subjects guessed that a person would not reject them, but then did, heart rate decreased more and took longer to recover to normal.
In related research it was found that those with low self esteem experienced higher levels of "social pain" and those with high self esteem consistently overestimated the number of times they would be accepted. Additionally those with high self esteem had less change in the areas of the brain that process acceptance and rejection, showing that if you have high self esteem you'll have a positive outlook that helps protect you from feeling the social pain from rejection.
How do we use this info to help us?
Exercise the old axiom, "don't worry, be happy", and for sure, think positive things about yourself :-)
Low self-esteem allows brain to make connections that allow for greater "social pain", when people are rejected by peers