Why did Hemingway write The Sun Also Rises?
Three things from the book that changed your perspective of the time period? (world war 1)
Night life for the wealthy classes of folks is represented in this novel in ways that I have been surprised to see continued through the end of WWII and a little beyond.
Comparing, say, Catcher in the Rye and Sun Also Rises, I am surprised to see that the same kind of night club culture persisted from the 1920s to the 1950s, with the implicit class distinctions involved still apparently intact.
After a trip to Spain with his wife, Hemmingway used people he met there as characters to write about the disillusionment that all felt after the Great War. Many Americans flocked overseas to get away from Prohibition and most of the writers took to writing for the newspapers or magazines there in Paris.
The second part of your question about the three things is a personal answer. In this time period, I believed that most people (unlike today) had good strong morals, good manners and used their time wisely. After the war, I was surprised how much they drank in the book. I first read this novel as a teenager, so the overwhelming amount of drinking always bothered me. (I was a good girl throughout high school...)
Another thing that bothered me was the uselessness. They seemed to do a lot of nothing. There was no ambition, no strive to make their lives better. They just spent their money with no purpose other than for their own enjoyment.
I suppose I was also bothered by his physical condition after being injured in the war. That his problem was impotence is not the problem, but his "injury" caused him to never be able to consummate his love/relationship with his love, Lady Brett. He could've lost a limb and might have been happier, but the war left many wounded behind, both physically and spiritually/emotionally.