Why do people develop loyalty toward the groups they join? Evolution? Reciprocity? Indoctrination? Either way, not unusual. Happens all the time in the militar

Why do people develop loyalty toward the groups they join? Evolution? Reciprocity? Indoctrination? Either way, not unusual. Happens all the time in the military ("no such thing as an ex-marine"), around football clubs ("once a red, always a red")... pretty much everywhere there are humans, especially men in high-pressure environments.