Existentialists realize we are all islands, connected only by such undependable links as small boats in a storm. They realize that “yes, that’s all there is.”
However, just like Christians believe you must be “born again” to be happy, existentialists have to achieve “authenticity” to be happy. Don Draper is miserable because he doesn’t know he has an existential perspective. Once he accepts his flawed perspective or once he becomes authentic, he will sing, “yes, that’s all there is, and that’s okay!”
Existentialism is not a philosophy. It is not a religion. It is merely a perspective, that colors our view of the world. For Don Draper, his perspective darkens his world and isolates him from the joy in that world. Once he realizes his perspective has merely discolored the world he sees, he will be “authentic” . . . and then he will be happy enough to enjoy the talented men and talented, beautiful women around him . . . and that’s okay too!