Why did Chandler Bing get into the advertisement field?

On Christmas Day, Chandler quits his job so that he can fly home to
New York to be with Monica. Monica helps Chandler secure a job in
advertising through an old college friend of hers. Much to Chandler's
dismay, the job is as an intern, which leaves him in the awkward
position of working alongside people who are significantly younger
than he. However, his more mature approach eventually pays off for him
compared to the other interns, and he secures a full-time job in the
business as a junior copywriter when he only expected to receive an
assistant position. The company concluded that he would be better
suited for a more senior role. (Source)