хурсандӣ

Хурсандии ту хурсандии ман аст.
Your happiness is my happiness.
Хурсандӣ ба мотам табдил ёфт.
Joy turned into mourning.

Bonus:
Synonym: шодӣ
The adjective, хурсанд, is made into a noun by adding the suffix ӣ.
хурсанд: happy, joyful
хурсандӣ: happiness, joy
Other examples of changing an adjective into a noun by adding the suffix ӣ:
ором, оромӣ: peaceful, peacefulness
сер, серӣ: full, fullness
миннатдор, миннатдорӣ: grateful, gratefulness
Here are a couple more examples. If the adjective ends in a vowel, the suffix is гӣ:
гурусна, гуруснагӣ: hungry, hunger
ташна, ташнагӣ: thirsty, thirst

---