With all respect,
In my opinion atheism by definition imply having a non happy life, for many reason, including:
1. It just teaches that there is no goal for life.
2. With no believe in judgment after this life...an atheist is more likely to commit mischief , and thus more likely to expect others to do the same...Hence atheists have a trust problem.
3. Atheism leads to Hedonism, which can only results in sadness.
In may opinion (and it's the opinion of Shia Islam), belief should start with reason. I mean God can be proved by reason...The problem with most atheist today is that they generally don't read the replies to their belief. Atheism is one of the oldest ideologies, and what current atheists repeat has in essence said thousands of years back...and has been falsified by theists...That's why religion has prevailed...to try to turn the clock back, you need to do your homework first ...understand you arguments, understand the replies , and come with something new.
That's why you don't find an atheist who is sure about his stance... On the contrary, true believers, have the internal piece and certainty of their beliefs which are built on reason...
In addition believers are on the safe side...let's support that there is no God...Believers have lived a happy life, anyway. However If an atheist will assume that there is God, he will just be in trouble.
Wish nobody will misunderstand what I wrote...I respect everybody...
As Imam Ali (a.s) who is the second man in Shia Islam said:
People are of two categories: either brethren in faith, or peer in creation.