Why? Because words about others reflect not only a person’s worldview, but also a person’s character, values, ability to empathize, or honesty. Someone can speak flatteringly about everyone around them, hiding envy or self-interest. Someone can criticize too often and unfairly, revealing inner anger or intolerance.
A person who knows how to respect and appreciate others, even if they are strangers or competitors, will probably treat you with warmth. And someone who constantly gossips and puts others down will sooner or later show similar behavior towards you.
This rule works like a mirror: what we see in words about others is a reflection of who we are. By listening and observing, you can not only understand your interlocutor, but also learn to better understand yourself.