I tend to disagree about atheists not being “happy.” This discussion could get psychological so to avoid that, I will bring joy and hope into the mix.
As a Christian, I have fought depression. My faith in God and my relationship to Him is often my only source of joy. My hope comes from thinking about when Jesus rules this world and our many children will once again be playing outside without fear and we will not have to ever lock a door again. Heaven is going to be so wonderful that I can’t even imagine what that life will be like so I stick to what I know will be here and enJOY it.
Other things make me happy. Ice cream, a wonderful photo, my silly alien looking rat terrier, thinking about living in the mountains again, and a white Christmas. Joy is what Christians have, anyone can be happy.