I can understand how they can be happy. Most people would be very happy to live a life in which you would be able to do WHATEVER you feel is right, regardless of what that was (as long as you weren’t arrested for it). Not believing in God frees people from the guilt and pangs of conscience that accompanies the belief that some Higher Power may hold you responsible for the choices you make. If you can convince yourself that there is no Higher Power, then you will believe that no one will judge the choices you make. In this way, atheism is sort of a faith, a deep hope and belief that there is no God.
If you don’t believe in God, then you live only in the here and now and have little concern about others and their circumstances beyond the amount of concern that the human laws and regulations insist you must have (for example, an atheist would avoid t murder, NOT because some “God” said that murder is wrong; rather, they would not kill someone because of the knowledge that they would be arrested and put in prison, losing their freedom.) They don’t worry about consequences or obligations beyond what society would require of them. They believe that if they are “good people” in the eyes of society, keeping human laws, then that is good enough. Atheists are also content in their belief that humans can solve every single problem on Earth and that whatever problems humans cannot solve are not worth worrying about. They don’t worry much about death and disease or intractable poverty or hunger because either they believe that humans will eventually solve those problems, or they believe that those problems are the evolutionary “lot in life” of people who are not the “fittest” and that the laws of evolution will take care of the matter soon enough. No point in caring or worrying about it at all.
Finally, they think of themselves as individual little “Gods” or “Goddesses” in that they believe that they, as imperfect human individuals, are the sole source of moral authority in their lives, that whatever their personal situational ethics dictates is what they do, and they alone have complete control over their own fortunes. Each one lives as they want to live with no outside sources of ethical rules (other than the laws of imperfect human governments). If it is not legally mandated, you don’t have to do it. If it “feels good” to you personally, then do it. They feel no obligation toward any universal source of authority and believe themselves to have attained ultimate freedom.
I can understand how some folks could think this is happiness.