It says “schar mitzvah: mitzvah [the reward for a good deed is another good deed].”  The reward of a mitzvah is a mitzvah, and the reward for joy is joy.

How does G-d pay you for being happy?  He gives you another minute of happiness.  So if this one minute of happiness is real, the next minute also will be.  But if you say “I’m so happy to be alive,” and the next minute you are walking around downcast, then there was something wrong. ~ R’ Shlomo Carlebach