It is argued that one way to achieve happiness in one's life is to always strive to live in the moment. Don't dwell too much on the past and try not to worry too excessively about the future. Instead, enjoy each moment to its fullest. I propose that atheism facilitates such an outlook on life.
I just returned from a vacation where I had the pleasure of meeting numerous interesting people from all walks of life including one couple who are very religious. After several days of jostling and teasing each other about our respective existential views (with utter respect), to their surprise they concluded, "that I was one happy atheist." I suppose that their surprise stemmed in part from the mistaken view that lack of religious belief would somehow yield a pointless and solemn existence.
This leads me to Horace's famous Carpe Diem quote. I contend that to seize the day and enjoy the magic of every moment is much more congruent with atheism than it is with religiosity. Please understand that I am in no way denigrating people's rights to their private beliefs. Rather, I am making a philosophical and existential claim that atheism is a much stronger conduit for carpe diem.
To the extent that most religions promise an eternal life, the here and now is simply less important, as one has an eternity to get it right (e.g., through redemption). Atheism suggests that there are no "do-overs" and no second chances in the afterlife (or in a reincarnated life). One cannot hope to be happier, more moral, more kind, and/or more fulfilled in the afterlife. It is all happening right now within our very short existence. Ultimately, an atheist knows that his/her legacy (in whichever way that it is measured) will be left behind during the here and now.
Of course, I do not wish to imply that religious people are any less happy than atheists. As a matter of fact, there are many life metrics (e.g., longevity) that improve by virtue of being religious. My only point is that seizing the day is more congruent with atheism than it is with religiosity. I realize that it might not be popular to write about such matters when the holidays are approaching so apologies for doing so. That said I do wish everyone a peaceful, happy, and healthy holiday season. Ciao for now.