Serving Hashem is only when it is b'simcha

קומי אורי - "Rise and Shine" (Yeshaya 60:1). The word "קומי" and the word "אורי" are two commands for it states "awaken and arise from the dust", because first one must arise from the dust and sadness and lethargy, and after this אורי "shine" -- and this אור (light) is the simcha, as it says "the Jews had orah [light] v'simcha v'sasson v'yikar"[...]

Even when a person's intellect agrees to serve Hashem, this is still darkness and he is [still] not doing the will of the Creator -- instead he's doing his own will and what he himself agrees to. Rather, the main thing in serving Hashem is to serve Him with joy b'simcha u-v'tuv levav b'rov kol.

Admor Hazaken, Neviim, p.180-81