Why are people unwilling to listen to "useful" principles? Because you're making them admit that they've been avoiding the truth all along, you're making them realize that the problem really lies within themselves, you're making them believe they have to change, and you're making them lose the sense of security that "I am right." Their resistance to your reasoning isn't because your reasoning is wrong, but because you've made room for rationality. This is human nature at work. Because principles are often counter to human nature. For example: delaying gratification, long-term thinking, restraining desires, accepting mediocrity, admitting one's limitations, not always seeking to be liked. All of these are very useful, but they don't feel good.