To "serve God in happiness" suggests that one should be joyous while serving God. According to the Baal Shem Tov and subsequent Hasidism, however, joy is more than merely an ideal state in which to perform religious acts. The joy itself becomes a necessary ingredient for all religious life, a primary religious category.
In the same way, sadness is also dangerous, for it is a confabulation of our evil side. When you are depressed, you are not only unhappy, your will is weakened, you are unable to act. You have literally lost the good fight.— Lawrence Kushner, Nehemia Polen, Filling Words With Light