The Lost Generation, as portrayed in The Sun Also Rises by Ernest Hemingway is a group of writers, artists and intellectual born ca. 1900, who are living through the 1920s, a post-World War I period also called "the Jazz Age". The group Hemingway portrays is a mix of some working writers (who did not inherit wealth, like Hemingway himself and his friend Bill), some aristocrats or other scions of wealthy families (Robert Cohn is a portrait of Harold Loeb, from the wealthy Guggenheim family) that clustered around an older generation including Gertrude Stein and Ezra Pound and Shakespeare & Co. book store in Paris, mainly on the Rive Gauche near the Quartier Latin.
Reasons for "the cause of their restless and ceaseless travelling and revelling nature` – The primary reason is that the exchange rate was good enough so they could afford to do so. In part, isn`t this just a very ordinary desire of young adults, just after university, to take a few years off to travel and party and explore the world and themselves? Although Hemingway, in this novel, tries to take himself very seriously, in many ways this is just a story of young people having fun, exploring their own identities, going through all those intense melodramatic relationships one has at a certain age, etc. before they settle down to real jobs and marriages.