I can understand trimming a beard being relatively easy and necessary, but actually getting a close shave is really annoying, not to mention dangerous back in the days of a straight razor. Why would any group of people just decide to start doing so? Answers for both facial shaving and body shaving would be great. Thank you.
u/voyeur324 has already recommended some earlier answers, focusing on women's body hair especially. For some more, you can read here about hygiene, including facial and body shaving in Ancient Egypt (the earliest example I found in previous threads) written by u/Bentresh. In this thread u/Celebreth has written about body hair removal in Ancient Rome, and here u/toldinstone focuses on facial hair in Rome and Greece (though he might have made a minor error, Pliny actually says (N.H. book 7, 59/211) that barbers first came to Rome in 300 BC and that "Africanus sequens" (probably Scipio Aemilianus) was the first Roman to shave daily). As for non-European women, the Crusader period Arab writer Usama ibn Munqidh mentions shaving pubic hairs as common among Arabs (possibly both men and women) but uncommon among "Franks", as discussed by u/AbouBenAdhem here and u/WelfOnTheShelf here.
/u/sunagainstgold has previously answered Were medieval women as hairless as the movies show it? When did women start to shave?
/u/mimicofmodes alias chocolatepot has previously answered When did it become the social norm/standard for women to shave their legs completely?
More answers remain to be written, especially about women outside of Europe.
See below for more answers about the 20th century