Some people feel inadequate when they see or hear about other people's good qualities and good accomplishments. Envy and comparison make people feel dark when others shine: “that woman is so gorgeous; I'm so ugly” and “he's so intelligent; I'm so stupid”.
Such thoughts – especially if framed as absolutes – can make people feel worthless. If you ruminate about what you judge as your weaknesses, and forget/invalidate your strengths, then you will only lower your own self-esteem.
If you are preoccupied with self-criticism or to diminish the success of another (e.g. “she's a bitch, she just got lucky and doesn't deserve it”), then you'll probably miss the opportunity to learn and celebrate. You will diminish your own motivation and capacity to shine on your own. And all of this will harm your personal happiness.
In contrast, when we are delighted by the good qualities and accomplishments of others, we feel sympathetic joy. If I am concerned for your happiness, then I will want you to have gifts and talents that make you succeed in life.
If you take pleasure in other people's qualities and accomplishments, you will increase your own happiness. You will be able to genuinely compliment them on their qualities and accomplishments, and they will feel grateful towards you and pay back the compliment later. And, we should recognize and show gratitude towards the virtues of others, because they make this world a little brighter.
This requires that we are aware of other people's qualities and accomplishments to fully appreciate and acknowledge them, or else well just take them for granted. Just as we want our own qualities and accomplishments to be acknowledged and appreciated, we should acknowledge and appreciate those of other people.
Neff, K. (2011). Self-Compassion.