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It boils down to lifestyle.
MINI people hang out and do MINI/Mini things together; it becomes family. BMW people try to impress each other by finding the stinkiest cheese in town and listening to bad jazz.
I know you are joking, but I have to dispell the notion that BMW fans (or lots of other car or motorcycle enthusiasts) do not have robust and friendly communities. The Mini community is fairly centralized and thus visible, but other groups and fans of other brands have not only been around a long time, but also are social, supportive and fun; some are more local vs national so are not as broad as say NAM, but still very active. I met my husband because of BMWs Since then I've delved into the Mini community, Ducati, Audi, Porsche, Smart, and more - and will enjoy others I'm sure