Existentialism is a Humanism is an essay written by Jean-Paul Sartre
in 1946. He basically tries to shake off the gloomy, negative labels associated with existentialism
and explain that, though many people are calling themselves existentialists and moping around, these people are poseurs
and should not be taken seriously. He affirms that there are many reasons to believe in existentialism and atheism
and that in doing so one does not automatically reduce onself to a chronically depressed maniac.
For younger readers this can be related nicely to a lot of music today and the general glamorous depression attitude a lot of people take and how that attitude makes it extremely hard to tell the people who need help from the people who need attention. True existentialists probably didn't talk about it all the time. True existentialists simply were so and did not need to affirm it daily.
I highly recommend the essay which can be found in it's entirety at: http://www.marxists.org/reference/archive/sartre/works/exist/sartre.htm (URL to be altered if need be.)