I think dress customs like that reflect a teenager's desire to have an identity independent from that of the adults to whom he has to answer - parents, fosters, coach, etc. It also reflects thinking along the lines of "I don't need to wear (fill in the blank) just because adults do and just because they say I should."
In my day, the things we hated/refused to wear were hats or caps in the winter, dress shoes, ANY shoes, straight-leg trousers, collared shirts, etc.
Talk about giving away my age!!!!! I preferred to freeze rather than put something on my head, even though I knew a cap made great sense in cold weather.
In the UK, where boys have to wear school uniforms, the first thing they do when they get out of school is pull their shirttails out, pull their ties loose, and walk down the street looking as disheveled as they possibly can - in jacket and tie.
It's all part of the journey of self-discovery that every boy goes through.