https://en.wikipedia.org/wiki/Narcissism_of_small_difference...
People want strict rules that they can follow that gives them a measuring stick by which they can gauge their own (and others) "worthiness", "purity" or "success". If we set specific boundaries, then we can more easily judge ourselves if we succeeded/failed or others if they are in/out. This type of thinking is most commonly found in diets. We decide which foods are "bad" and which are "good" so we know whether or not we have achieved our goals. It's easier to know when you "fell off the horse" when you ate a food previously deemed "bad". It's not so easy to make judgements on ourselves and others when the rules are not so clear-cut.
I think many are afraid of looking to ourselves as the deciders of our own realities/moralities. We prefer to look outside to leaders/gurus/experts to tell us what the truth is.
I'd love to know if there is a handy word/term for this phenomenon where an odd amount of energy is spent opposing those people/views which are more similar than less similar.