Oomis: Ta’anis Esther, if I recall correctly is to commemorate the three day fasting of the Yidden in Shushan, and the fasting of all the Yidden the day before the want out at war to protect themselves. We fast to memorialize the concept that HaShem is ???? ?????? ????. That’s not really a contradiction to merriment.
It is those sad occasions, whose fast commemorates mourning, that are a contradiction to making merry.
(I think in the Bais Hamikdash, the Leviim played music on Yom Kippur, on Tisha bAv they didn’t)