Parashas Ki Savo · 28:47–48 (Sifrei / midrash) · Not cited by Torah Temimah
Sifrei and parallel midrashim on “תחת אשר לא עבדת את ה׳”.
English: Because you did not serve Hashem your God with joy — therefore you shall serve your enemies… in hunger and in thirst and in nakedness and in lack of everything.
Custom (causal link drawn by Chazal between joyless service and subjugation).
Joy in avodas Hashem is not an optional flourish; its absence is treated as a decisive cause of the reversal into forced service of enemies.