Researchers have found that older children and adults who are sore losers worry about what others think of them if they don't win, or feel that winning is what makes them good people. (Interestingly, this confusion of doing something good with being someone who is good is typical of how toddlers think.)
It seems to me that the problem here is the extent to which people see winning versus losing as some sort of test of their self-worth. If, as appears to be the case, a person involved in a competition of some sort requires a victory in order to feel adequate or competent, the likelihood that they can tolerate a loss is significantly lessened.
