Repentance, like all mitzvot, must be fulfilled with joy, even though it involves deep pain. Such an important mitzvah as repentance, moreover, which has the power to rectify faults in all the mitzvot, must certainly be fulfilled joyfully. Indeed, repentance itself is paradoxically compounded of both pain and joy: the sinner is able to return and cleave to G‑d, and there is no joy greater than this.

Repentance, then, has two components: bitter regret for the past, and a joyful resolution for the future.

(Likkutei Sichot, vol. 2, p. 389)