Rosh Hashana is a Yom Tov and there is a chiyuv simcha. It is also the Yom Hadin. Therefore, we tone down the expressions of simcha. The shiu explains the nature of the simcha on Rosh Hashana. The Rav zt'l explains that tekias shofar is a form of tefillah and the tefillah of the shofar blends in with both aspects of the day. This yesod is dveloped at length.