When civilizations fade and even your last lingering ideals have turned to dust, you will look at the world and ask, "What did I ever do?" For everything will be gone, and it is only human to wish to see what you have done. To wish to see something with your name on it, your mark.
And the little girl will take your hand. She will take you to the chatting, smiling crowd. And she will say, "This."
You'll understand. "But happiness is fleeting. I can't have done any more than just made them feel better for a few seconds." Even if you have cured cancer, you will feel like this, if only for a moment.
The little girl will smile. "Silly. Happiness echoes. And people just keep adding noise!"
Then she'll skip off to another, while you stand in the smiling crowd until you go to someone or someone comes to you. It'll only take a moment. After all, after everything's gone, what else is there to do but get to know each other?
And you'll smile, and maybe you'll walk off. And maybe you'll become one of the little ones, going out and finding those who have been alone too long, learning how to see when someone just needs a moment. Or maybe not. In a truly infinite world...