Cialdini and his colleagues (1976) counted the number of college insignia t-shirts worn by university students on the monday after their teams played football. these researchers found that students were more likely to wear such shirts after victories than after losses. these findings suggest that

I believe the answer is: in-group identification is a source pride and self-esteem.

in-group identification would make most people develop the feeling of being accepted and acknowledge by other members of the group.
This would tend to increase their pride and self-esteem and make them able to develop a sense of belonging.