Glover's premise is that nice guys have been conditioned by their childhoods and by society to believe that they will be successful only if they make everyone happy, and never cause any problems for others. However, this desire for approval is countered by self-loathing. In other words, nice guys want approval, but don't think they deserve it. This creates internal frustration--nice guys never try to obtain what they want in life. In addition, the nice guy's desire to obtain approval from everyone (especially women), causes him to actually behave in very un-nice ways. This includes dishonesty (about themselves) and passive-aggressive behavior (to get what they want without asking for it).
Dr. Glover's prescription involves getting nice guys to recognize that their needs and desires are important, and that to make others happy they must first learn to make themselves happy. One of the primary ways advised in the book to remedy this is for nice guys to learn to embrace and develop their masculine traits, instead of fearing and suppressing them.