As it were, the Holy One, blessed be He, has not yet rejoiced in His world—and shall human beings seek to rejoice in His world? “The LORD rejoiced in His works” is not written here, but rather “Let [the LORD] rejoice [in His works]” (Psalms 104:31). And when will He rejoice in the works of the righteous? In the time to come.
When God Will Rejoice in the Works of the Righteous
Midrash Tanchuma Buber, Achrei Mot 3:2
כביכול הקב"ה לא שמח בעולמו ובני אדם מבקשין לשמוח בעולמו, שמח ה' במעשיו, אין כתיב כאן, אלא ישמח [ה' במעשיו] (תהלים קד לא), ואימתי ישמח במעשיהם של צדיקים לעתיד לבא.