How sportswear become an integral part of everyday dress?

I think that most of it has to do with comfort and practicality especially if you consider that tennis shoes were originally for, well tennis. They are comfortable, so most people where them all the time now. Likewise, t-shirts were not always considered everyday attire, but because they are comfortable, versatile and inexpensive, people where them every day. It makes sense. Why should people own clothes and then only where them for a specific activity? If you are speaking of people who wear jerseys and the like, I think that is because they are supporting a team or player that they like. And also because jersey can be expensive, which makes them a bit of a status symbol.  more