The most powerful man in the U.S. health care industry is telling me about the challenges of parenting an unruly adolescent. At least it sure feels that way as Stephen J. Hemsley, the CEO of UnitedHealth Group (No. 17 on this year's Fortune 500), describes his efforts to tame his company's previously selfish and belligerent behavior. Hemsley is a former accountant with a reputation in the business as a taciturn, obsessive numbers man and not much of a people person. But when he sits down with me in a nondescript conference room in UnitedHealth's headquarters in Minnetonka, Minn., for the first face-to-face interview he's granted in his six years as CEO of the nation's largest health insurer, Hemsley says hardly a word about premiums or actuarial data. Rather, in the soft tones and measured sentences of a brainy college professor, the 60-year-old executive talks about more abstract concepts such as fostering collaboration and building EQ.