How does being fashionable make someone confident?

It can depend on the person, but I think for the most part it just makes you feel secure about the way you look, put yourself together, and present yourself. If you're fashionable, it is assumed that you know how to make yourself look good, meaning not only the clothes you wear look great together and have an artistic feel, but they complement and enhance your body, good features, and your personality. It's another way of expressing yourself and showing a collected face to the world.