Show affluence - that's a good one.
It's only natural to show off, every animal does it, it's natural.
People follow fashion because everyone has the desire to fit in, be liked, be attractive etc.
As for why we want to be 'different', we all want to be individual & believe that we are not influenced by anything other than our own likes & dislikes - obviously this is doomed to fail.
Why do we want to 'be hard' - survival. It's in an animal's best interest to be feared. Obviously we are civilised now, but we are still influenced by the parts of our brains which deal with 'primitive behaviour'.
Basically, if you look hard enough, you'll find mostly all our behaviour & desires parallel those of other animals. There is much of us that is predefined by thousands of years of human life. (Obviously there are those who do not wish to believe this, see the 'individual' bit.)