VLADIMIR
Let us not waste our time in idle discourse! (Pause. Vehemently.) Let us do something, while we have the chance! It is not every day that we are needed. Not indeed that we personally are needed. Others would meet the case equally well, if not better. To all mankind they were addressed, those cries for help still ringing in our ears! But at this place, at this moment of time, all mankind is us, whether we like it or not. (2.526)
Vladimir decides that he and Gogo represent all mankind only once they are asked for help (in this case, by Pozzo). Didi is only able to assign meaning to his life when he is depended on, which is why he needs Estragon as much Estragon needs him.
VLADIMIR
But that is not the question. What are we doing here, that is the question. And we are blessed in this, that we happen to know the answer. Yes, in this immense confusion one thing alone is clear. We are waiting for Godot to come— (2.526)
Vladimir uses the act of waiting for Godot to assign meaning to what is otherwise an entirely meaningless series of actions and interactions.
VLADIMIR
All I know is that the hours are long, under these conditions, and constrain us to beguile them with proceedings which—how shall I say—which may at first sight seem reasonable, until they become a habit. You may say it is to prevent our reason from foundering. No doubt. But has it not long been straying in the night without end of the abyssal depths? That's what I sometimes wonder. You follow my reasoning? (2.535)
All right, Vladimir’s little speech here is tricky. What he’s saying is, when you’re doing absolutely nothing every day for your entire life, time moves pretty slowly. The only solution, then, is to fill up your time with a series of actions. At first these actions seem "reasonable"—you take off your boots, you put on a hat, you converse or argue. But as time goes on, these daily actions become habit, and that’s when it starts to get a little absurd. In other words, take off your boots once, that makes sense. Take them off and put them back on twenty times a day for a decade, and that no longer makes any sense.