What is the significance of the word "Tachas Lo Avadta es Hashem Elokecha be'Simchah"?
R. Bachye: It means that the troubles will come upon Yisrael because, although they served Hashem, they did not serve Him with joy. Serving Hashem with joy is a Mitzvah in and of itself, and one is therefore punished for failing to do it.
Arachin, 11a: It refers to the Shir of the Levi'im, and teaches us that the primary Avodah is be'Peh - and the instruments are only secondary. 1
See Torah Temimah, note 30.
What are the connotations of "Tachas Lo Avadta es Hashem Elokecha ? me'Rov Kol"?
Rashi: It means that the troubles will come upon Yisrael because, when they had everything, 1 they did not serve Hashem.
Orchos Yosher p.85, citing the Ari Zal: One must serve Hashem with Simchah greater than all worldly pleasures, fine gold and gems.
Sifsei Chachamim: As if the Torah had written 'be'Rov Kol'.
