Losing an advantage, ego-related conflict, relationship friction, power struggles

Not being invited to an anticipated event (a birthday party, gala, wedding, etc.) 
A best friend spending time with a new love interest instead of the character
An ex's friends taking their side and disassociating from the character
Being an outsider in a community that embraces a different lifestyle (outdoor enthusiasts, vegan, swingers, doomsday preppers, nudists, a religious sect, etc.)
A character being excommunicated from a religion they identify with  
Being denied access to a social club or community—due to economic status, ethnic background, or education level, for instance
Being denied membership to a fraternity, gang, secret society, or other organization 
Marrying into a family and being treated like an outsider
Moving to a new school or community and being excluded socially
Decisions being made by other team members or co-workers behind the character's back

Being viewed as a third wheel by peers
The character going out of their way to try to fit in
Awkward conversations when the character runs into members of the excluding group
A loss of status
Having to attend events where the character knows they won't be treated as well as everyone else
Having to build a new community or event to replace the exclusionary one
Finding it hard to focus because of hurt feelings
Having no one to make special plans with
Having too much time on their hands
Time spent searching for new social groups, communities, or religions to fill the void
Unhealthy coping (eating when lonely or rejecting good people who try to include the character, for example), creating new problems
Having to suck up to the group to be included
Wasted time spent trying to find out why the exclusion is happening