In order to rejoice at the happiness of others and share it with them, the Buddha suggested to practice the following phrases toward others:
"May your happiness not leave you,
may your good fortune not diminish,
may your joy continue."
When people practice with these phrases, over time it helps them to recognize and be grateful for their own happiness, good fortune, and joy; it also contributes to them rejoicing at the happiness of others.
— Martine Batchelor in Let Go