I have been wanting to address this issue for quite some time now, I guess now that opportunity presents itself, I should attempt to tackle it.
What defines a society? A society is defined by the people that exists in it. And so who defines social norm? A social norm is defined by the values that majority of the people in the society have agreed upon to act by. Any deviation or transgression is frowned upon or may result in a different treatment from the majority. Failure to adhere to these norms may also result in punishment, physically or mentally, exclusion being the most feared in my age group. Or least that is what I feel.
And those individuals that have been excluded from the society may eventually group together to form a minority, a cult. Sometimes these groups of minority attract so much attention that they are partially accepted into the society, not as an exclusion, but as a sub-culture, something that people can tolerate, perhaps out of of novelty, curiosity, but not embrace it.
And how do these groups of minority ( cosplay etc ) attract so much attention and separate individuals that it manages to find its way back into the society? I think it is because these groups offer something irresistible that reflects back onto these individuals-acceptance.
You have to agree that most of our actions may seem independent, unrelated to anybody and are acted out entirely upon our free will. But these actions are actually influenced largely by how people may think about ourselves. I do not know how to put it in a more defined manner that is less contradicting. One must agree to a large extent that our actions are governed by both our desire and the social norm, or as I would term it, the socially acceptable behaviour.
What about new trends, new behaviours, new rules that spring up, previously not accepted, or are frowned upon? My guess is that they must have sprouted out from people with an unusually high social ranking. High social ranking-people with a reputable name, well known figures in various industries, well connected, wide network. So lets say they start something so radical, it seems almost absurd at first ( like men going for facials ), but because of their special status, people around them are willing to tolerate it, waiting around to see if there are any repercussions from the society. In addition to their octopus like network, people whom yearn to establish themselves on the same level as these people with usually high social ranking, see it as a bench mark to qualify to enter that "level", to be mentioned together with them, start to embrace these practices.
Eventually, these practices trickle down to the rest of the society and people catch it like puenomia, a few cases present themselves, harmless, and then, boom, spreading like wide fire. And that becomes part of the social norm.
So I guess we will be chasing, updating and following social norms for the rest of our lives.