There is probably nothing more satisfying than being able to think about life's great questions without the restrictions created by religion and other superstitions. If someone inquires about your views, you have an answer. If you don't have an answer and are at fault, you can easily adjust your personal philosophy to better fit with reality. There are no imaginary chains.
Religiously minded people will often ask how someone can be happy without a belief in a god. Any atheist reading this blog can relate to how confusing that question can be. In our minds not believing in a god is like not believing in Santa. Thoughts of Santa filled many of our childhood minds with joy but we were only sad for few moments when we realized or found out Santa is not real. Religious views of god is just Santa for adults. Thoughts of god make many people happy, but that doesn't mean they would be forever sad if they found out god is imaginary.
I also find the question confusing because belief in god doesn't make everyone happy. It seems that for quite a few that belief just gives them a reason to carry on a life they don't like in hopes that when it is all over they'll be rewarded in another life. That's actually a very sad existence if you think about it...this life is not about this life but is about the next.
"When I finally came out of the closet and allowed myself the freedom of being an atheist, it was like a butterfly emerging from its chrysalis. I could read anything I wanted. Ask any questions I wanted. Challenge anything that didn’t make sense to me. And after being the equivalent of a worker ant for so long, this was heady stuff indeed! I am still exhilarated by it. A chained intellect, as demanded by religion, is a pitiful thing. A free intellect is a thing to be treasured.- Judith Hayes - The Happy Heretic
While many things cause joy for individuals, if there is anything specific to freethought that makes people happy it is what was described in the above quote. The complete freedom to simply be the person you want to be without worrying about conforming to pre-packaged views is a lasting happiness.