Most marriage therapists and relationship books warn against "mind-reading," which means assuming that your partner knows what you want. With some couples this is good advice. But one of the reasons that marriage counseling usually fails in relationships with chronic resentment, anger, or emotional abuse is that your partner can read your mind when your mind is negative.
If you think or feel "nag, jerk, selfish, liar, loser, irrational, irresponsible, narcissist, borderline, or abuser" or make any other negative attribution in proximity to an intimate partner, it will certainly be communicated, no matter how positive the words you use; indeed, even if you say nothing at all. Remember, our species communicated sentiment through body language and emotional demeanor long before the development of language. Even low-grade resentment and anger -- irritability, impatience, defensiveness, chilliness, agitation, annoyance, and contemptuousness -- are clearly communicated without words.
All of this means that if you try to communicate verbally without changing how you think and feel, you will seem to people you love to be saying one thing and thinking another, which will make you seem manipulative, if not dishonest. You will almost certainly get a defensive or otherwise negative response, even if your words are entirely positive.
In contrast, getting in touch with your core value changes the way you see yourself and the people you love. It makes you focus on what is more important to you, connection or punishment, compassion or resentment, growth or the status-quo.
At the very least, activating your core value before you attempt to communicate allows you to be more authentic with loved ones and reduces the chances of a negative response from them. But even if it fails to improve your interaction, you will feel more confident and genuine within yourself by remaining true to your deepest values.