Depression Leads to all Aveiros:
Sefer Divrei Bina (Parshas Tzav) cites a hint from the word ‘Bereishis’ to the concept of serving Hashem with joy. He writes as follows:
One should always serve Hashem happily, as is stated (Tehillim 100:2): ‘Ivdu es Hashem b’simcha’ (Serve Hashem with joy). Rav Nosson Dovid of Shidlovitze zy’a understands this pasuk to mean that simcha is a form of avodas Hashem in and of itself.
Chazal say (Sotah 3A) that one does not sin unless a ‘ruach shtus’ (spirit of foolishness) enters him. It is known that the cause of a ruach shtus is sadness, as we see that all mental instability starts with depression. We thus see that sadness is the root cause of all sin.
This concept is hinted to by Dovid Hamelech with the words (Tehillim 16:4): ‘The sorrows of those who hasten after another (deity) increase.’ This can be understood to mean that if one has sorrow, he will hasten after other deities (a euphemism for all sins).
Just like sadness is the root of all sin, joy is the root of all avodas Hashem. Thus, Dovid Hamelech tells us to serve Hashem with joy, as one can only properly serve Him if he is happy.
This is also hinted to in the verse (Tehillim 105:3): ‘May the hearts of those who seek Hashem rejoice.’ This can be translated to mean that if one wants to seek Hashem, he first must be happy.
There is also a hint to this in the word ‘Bereishis’, which can be a backwards acronym for ‘Tamid yihiye samach im rotzeh b’boro’ (One must always be happy if he wants to be with his Creator).