For most of human history, men have had beards. And it is easy to understand why. Cave men had beards because they had no choice -- they lacked any kind of blade to shave their beards off.
Many religions also have prohibitions against shaving. For example, in Leviticus 19:27, the Bible contains a specific prohibition against shaving your beard and the hair on the sides of your head. Some orthodox religions still practice this today.
Once metallurgy has been refined in any civilization, however, the technology of knives and scissors follows soon after. These cutting tools become more and more refined, and these refinements lead to the development of the razor -- the sharpest knife possible. With a very sharp knife, it is possible to begin shaving.
Even with these developments, however, men preferred beards. This may be because shaving with a straight razor is a somewhat dangerous activity better left to a professional. Unless you live in a city and are wealthy, being able to find and afford a shaving professional is difficult. And so, all the way up to the 20th century, beards were fashionable and most men wore them.
But during World War I in the United States, that all changed. And there were two reasons for that change:
- Gillette had released the "safety razor" in 1901, and it was steadily gaining popularity because of a massive ad campaign. The safety razor made it possible and inexpensive for men to shave daily.
- Soldiers in the United States army were required to shave.
Certainly one reason for shaving during WWI is the fact that it was the first war to see chemical agents used on the battlefield. Soldiers had to use gas masks for the first time. In order for a gas mask to fit properly, you need to be clean-shaven. The army bought millions of Gillette razors and blades to make shaving possible.
When all of the soldiers returned from WWI with their clean-shaven faces, they were heroes. They appeared in their home towns, and they also appeared in newsreels in the new movie theaters that had sprung up everywhere. Combined with ad campaigns from companies like Gillette, it became the fashion to be clean shaven. Between 1920 and 1960, beards were definitely unfashionable. That taboo has eased somewhat since the 60s, but it is still far more common for men to shave than not. And as you can see, it is strictly a fashion statement, and largely the result of advertising by companies like Schick, Norelco and Gillette.
Or, to put it another way, no one makes any money if you have a beard...