1. Like all categories it is powered by technology. The category only really started with the invention of the Spinning Jenny and the introduction of mass produced but very harsh to the touch trousers. A softer inner lining to prevent excess rubbing made sense and only the upper classes could afford silk. So cotton seemed a reasonably affordable, soft alternative to chaffing.
2. But driven by social need. Nylon, elastic and other innovations changed underwear but in the end, and strangely for an 'under garment', fashion and self perception have driven design preferences.
3. Marketing is always about creating demand. Whiter than white underwear originated when detergent makers convinced housewives their own reputations were judged by the cleanliness of their men's boxers and Y-fronts. Hence the global variations of 'put on clean underwear in case you get hit by a bus'.
4. Women define it. A few years ago it was estimated that 70 per cent of all men's underwear was purchased by women. Mothers who can't help themselves (come on guys we all know if mum did not buy us fresh undies no one would) or girlfriends (who else would have purchased that pair of g-string pink frighteners that are still hidden in the back of the drawer) or wives (and given there is no one we fear more we of course wear and smile).
5. Because sex does sell. Nothing changed men's underwear more than the introduction of the Pill and the swinging sixties. The myth of the sexual revolution and the possibility of the pick-up bar led to the magnificent array of colour, shape, shaping and style we see on the shelf today. And of course in 1991, Marky Mark, Kate Moss and Calvin Klein turned over 60 years of fashion to re-introduce a sexier boxer.