This quote suggests that being kind and considerate towards others is not only a good thing to do but also makes you a more likable and admirable person. When you treat people with respect and show genuine care for their feelings, it tends to create positive impressions and fosters good relationships.
Think of "cool" in this context as being someone that others want to be around. It's not about fashionable clothing or popularity; it's about having a warm and friendly attitude. When you're nice to others, you often attract kindness in return, which can make your social interactions more enjoyable.
Furthermore, being kind isn't just about appearances; it's about creating a positive impact on the world. When you're nice to others, you contribute to a more harmonious and pleasant environment for everyone. So, being nice not only makes you "cool" in the sense of being well-liked but also contributes to a better and more compassionate society.