Being fashionable means being able to shame and deride those who don't follow arbitrary rules of dress, and have society at large totally back you up for doing so.
Fashion is an expression of society's collective superiority complex - people are insecure, so to compensate for or conceal their insecurity, they invent reasons to dislike and abuse others for trivial reasons, and to convince themselves that they're better than everyone else.
Not happy in life? That's okay - you can smother your emptiness and despair in a thick coating of schadenfreude by finding completely insignificant differences between yourself and others and exploding them into grand delusions of worth and value!
Do you lay awake every night feeling lost, confused, hurt, alone, and angry? That's okay! Some five year old boy you passed on the street was wearing pink shoes, and that let you feel good about yourself for being so very "fashionable" in comparison! In fact, you felt so good about yourself, you just had to make the young boy feel miserable about himself to maximize the contrast!
Remember - the more miserable you make other people, the happier you are in comparison!
Fashion! Because misery needs the help!