Interesting. Going into Yom Kippur with the express goal of being happy, is really actually a nice way to experience Yom Kippur. Also, we know that Mosheh Rabbeinu tells us that all the bad things we have coming to us are because we didn't serve HaShem in happiness!
In the eyes of this new Torah we can look at Sukkot, called "The Chag" by Chazal, in a new light. A Chag is different from Shabbat in that we need to celebrate a Chag by being happy. That's why we say Chag Sameach, but not Shabbat Sameach.
We go almost straight into Sukkot after Yom Kippur as a means of showing HaShem: "See? I really internalized the teshuvah of Yom Kippur, I'm serving You through Simcha!"
So by all means, may have have a great big party this Sukkot!!
[Now all of a sudden the question "Why does simchat beit haShoeva happen on Sukkot?" (something I've always asked myself) doesn't seem like a question at all.]