Principles are ways of successfully dealing with reality to get what you want out of life.
Ray Dalio, one of the world’s most successful investors and entrepreneurs, cites principles as his key to success.
Emotions tend to heat up during most disagreements, especially when the subject is someone's weaknesses. Speak in a calm, slow, and analytical manner to facilitate communication. Put things in perspective by reminding them that their pain is the pain that comes with learning and personal evolution--and that knowing the truth will put them on the path to a much better place. Consider asking them to go away and reflect when they are calm, and have a follow-up conversation a few days later.
Ultimately, to help people succeed you have to do two things: First let them see their failures so clearly that they are motivated to change them, and then show them how to either change what they are doing or rely on others who are strong where they are weak. While doing the first without the second can be demoralizing to the people you are trying to help, doing them both should be invigorating, especially when they start experiencing the benefits.