Carl Sagan's message about the "pale blue dot" is, I think, the single most beautiful and important piece of writing I've ever encountered.
Here's how it begins:
Look again at that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every "superstar," every "supreme leader," every saint and sinner in the history of our species lived there—on a mote of dust suspended in a sunbeam.
(And there's much more. I recommend reading all of it, multiple times.)
Sagan wrote this passage after requesting NASA turn Voyager I around, so that, as it left our little system behind, it could capture a photo of Earth. It did, revealing Earth as a faint pixel in the dark. To Sagan, the photo underscored just how unique, how rare we are. How improbable our existence is.
I often revisit this passage when troubled, or even when things are going my way. This passage reminds me how gloriously small I am, my concerns or accomplishments, my worries or needs. It's such a valuable gift of context and perspective, and of wonder.