Answer by taking a quote from the following article:

Sir Pelham Grenville Wodehouse  (; 15 October 1881 - 14 February 1975) was an English author and one of the most widely read humorists of the 20th century. Born in Guildford, the son of a British magistrate based in Hong Kong, Wodehouse spent happy teenage years at Dulwich College, to which he remained devoted all his life. After leaving school, he was employed by a bank but disliked the work and turned to writing in his spare time.

A third milestone in Wodehouse's life came towards the end of 1915: his old songwriting partner Jerome Kern introduced him to the writer Guy Bolton, who became Wodehouse's closest friend and a regular collaborator. Bolton and Kern had a musical, Very Good Eddie, running at the Princess Theatre in New York. The show was successful, but they thought the song lyrics weak and invited Wodehouse to join them on its successor. This was Miss Springtime (1916), which ran for 227 performances--a good run by the standards of the day. The team produced several more successes, including Leave It to Jane (1917), Oh, Boy! (1917-18) and Oh, Lady! Lady!! (1918), and Wodehouse and Bolton wrote a few more shows with other composers. In these musicals Wodehouse's lyrics won high praise from critics as well as fellow lyricists such as Ira Gershwin.  Unlike his original model, Gilbert, Wodehouse preferred the music to be written first, fitting his words into the melodies. Donaldson suggests that this is the reason why his lyrics have largely been overlooked in recent years: they fit the music perfectly, but do not stand on their own in verse form as Gilbert's do. Nonetheless, Donaldson adds, the book and lyrics for the Princess Theatre shows made the collaborators an enormous fortune and played an important part in the development of the American musical. In the Grove Dictionary of American Music Larry Stempel writes, "By presenting naturalistic stories and characters and attempting to integrate the songs and lyrics into the action of the libretto, these works brought a new level of intimacy, cohesion, and sophistication to American musical comedy." The theatre writer Gerald Bordman calls Wodehouse "the most observant, literate, and witty lyricist of his day". The composer Richard Rodgers wrote, "Before Larry Hart, only P.G. Wodehouse had made any real assault on the intelligence of the song-listening public."  In the years after the war, Wodehouse steadily increased his sales, polished his existing characters and introduced new ones. Bertie and Jeeves, Lord Emsworth and his circle, and Ukridge appeared in novels and short stories; Psmith made his fourth and last appearance; two new characters were the Oldest Member, narrating his series of golfing stories, and Mr Mulliner, telling his particularly tall tales to fellow patrons of the bar at the Angler's Rest. Various other young men-about-town appeared in short stories about members of the Drones Club.  The Wodehouses returned to England, where they had a house in London for some years, but Wodehouse continued to cross the Atlantic frequently, spending substantial periods in New York. He continued to work in the theatre. During the 1920s he collaborated on nine musical comedies produced on Broadway or in the West End, including the long-running Sally (1920, New York), The Cabaret Girl (1922, London) and Rosalie (1928, New York). He also wrote non-musical plays, including The Play's the Thing (1926), adapted from Ferenc Molnar, and A Damsel in Distress (1928), a dramatisation of his 1919 novel.  Though never a naturally gregarious man, Wodehouse was more sociable in the 1920s than at other periods. Donaldson lists among those with whom he was on friendly terms writers including A.A. Milne, Ian Hay, Frederick Lonsdale and E. Phillips Oppenheim, and stage performers including George Grossmith, Jr., Heather Thatcher and Dorothy Dickson.

What happened in 1915?
Jerome Kern introduced him to the writer Guy Bolton, who became Wodehouse's closest friend and a regular collaborator.