In Waiting for Godot, Beckett writes: "The tears of the world are a constant quantity. For each one who begins to weep somewhere else another stops. The same is true of the laugh. Let us not then speak ill of our generation, it is not any unhappier than its predecessors. Let us not speak well of it either. Let us not speak of it at all. It is true the population has increased."
Ultimately, "all will vanish and we'll be alone once more, in the midst of nothingness!" It's the existential angst. In an absurd and meaningless universe, where and how do we find the reason to keep living?
In the last line of The Unameable, he leaves us with: "Where I am, I don't know, I'll never know, in the silence you don't know, you must go on, I can't go on, I'll go on."