The Torah says: וּב ח ר ת ֹב ח י יםֹֹנצביםֹלֹיט Chose life! Doesn’t everyone want to live? Not really! All negative and critical types of people have destructive tendencies. This is actually a major common Yetzer Hara! The Velt says: "You can be as happy as you want to be!" Sadness and complaining has a certain appeal that the Yetzer Hara creates, and it hampers your growth and productivity in a big way, Rachmana Litzlan.
A person doesn’t realize how the Yetzer Hara tricks him throughout his life, and he doesn’t realize what an actual Geshmak he may have in being a “failure” or being “weak” or “incapable”. This could be a reason why Tumas Meis (Tum'ah that comes as a result of contact with a corpse) is so severe, and whatever touches a Meis is also Tamei, because a person has to be a: בוח ֹרֹֹב ח י ים person who choses life! מ י םֹֹחַיִים ֹֹא לֹכ ל יֹ Live water! Live! Enjoy! This cures Tum’ah.
Rabeinu Yonah in Mishlei and the Rambam speak about the tremendous Inyan of being B'simcha. Why do we need a special Inyan to be B’simcha? Doesn’t everyone want to be happy anyway? We see that there is a special Yetzer Hara not to be B'simcha, and therefore we need to be told to be B'simcha (or else)!
Rabeinu Yonah: Obligation to serve Hashem with joy. The joy in mitzvos is a fundamental part of avodah.
Rambam: The joy a person has in doing mitzvos and loving Hashem is a great avodah. One who withholds himself from this joy is deserving of punishment.
