Perhaps it is an assumption but I would suppose that Hashem would prefer us to serve him through simcha rather than through trepidation. There is certainly a virtue in ahavas Hashem over yirah. Ahava is more easily channelled through simcha. Happiness can vastly increase our avodah, why not daven for that? Neviim do not have nevuah when in a solemn state; even the loftiest service requires happiness.
We pray for many things but not directly for happiness. For yom tov amida we mention simcha (etc.) but it doesn't appear to be some sort of goal (from the text of the amida) on a day to day basis. Why?