Other people may be in a group or country that they feel like they don't belong in but they fit in. For example, a country like Iraq. If an Iraqi doesn't fit in because of the war they might move, but they may not because they belong in that society. Because if they go to U.S. or China (example) they might just stand out and they might be worried about that.
I know where I belong, because I chose by what I feel is right and by the place or the group's personalities. I like being in the U.S. because the people around me fit in my example of my culture. Its more modern and I fit in pretty well. The group of friends I hang around have the closest to my personality.
That is what makes a person belong. It is by what they choose and what they are like in the group.