The Passuk says that we are punished “Tachas Asher Lo Avadta Es Hashem Elokecha B’simcha Uvtuv Levav”. HKBH wants us to be happy when we perform His Mitzvos. That doesn’t mean there is a Kiyum in randomly being happy otherwise. At some points we should be happy with life and some points in life call for more solemnity.
See the Aruch Hashulchan 551:1-4.