Why is the Jewish New Year so serious?

Rosh Hashanah is celebrated as the Jewish New Year but though it is a New Year, we do not make any wild parties or celebrate. It's not that Jews don't celebrate, we have other holidays when we party like Simchat Torah and Purim. Why then don't we party on Rosh Hashanah? Instead we spend much of the day praying in the synagogue. A discussion about the Jewish New Year, what it means and why it is so serious.