Laugh and the world laughs with you. (Weep and you weep alone.)
This saying originally comes from the Roman poet Horace, who wrote ut ridentibus arrident, ita flentibus adsunt humani voltus, or "human faces laugh seeing those who laugh, and correspondingly weep seeing those who weep." Other translations give us "laugh and the world laughs with you; weep and the world weeps with you." Okay, so that first part looks familiar, but we don't use "weep and the world weeps with you" anymore. Instead, we have another poet to thank for the second part. In 1883, Ella Wilcox wrote "Solitude" and gave us the version we tend to use today:
Laugh and the world laughs with you;
Weep, and you weep alone;
For the sad old earth must borrow its mirth,
But has trouble enough of its own.
What Wilcox and Horace mean is if we remain positive and cheerful, people will be drawn to us and our good humor—everyone likes a good chuckle. If we're seriously melancholy, though, Wilcox warns that people will avoid us like the plague.
Putting this saying to work in your fanfiction should be quite simple since both it and its sentiment are familiar. So you can use it to do things like draw attention to a character's sunny disposition—or to wryly point out that someone's being a sourpuss:
The Disreputable Dog charged ahead of the group and then, just as Mogget drew even with her, flopped belly-first into a large puddle. Mogget yowled and leapt backward, but not fast enough to keep muddy water from turning his white coat gray.
"Oh, yes," he hissed as the Dog opened her mouth in a satisfied grin and Lirael smothered a giggle with her hand. He glared at them both. "Laugh and the world laughs with you, Horrible Hound."
The Dog grinned wider. "Looks like it. Even Prince Sameth is smiling behind you, Mogget. Weep—or glare, I suppose—and you glare alone."
All the world loves a lover.
We can thank Ralph Waldo Emerson for this little gem, which comes from his essay on love (1841), though he said it a bit differently in the original text:
All mankind love [sic] a lover. The earliest demonstrations of complacency and kindness are nature's most winning pictures. It is the dawn of civility and grace in the coarse and rustic.
This saying means that seeing people in love makes other people happy. Love is a warm, fuzzy thing that spreads good feelings, just like laughter does in our first saying. And love tends to bring out the best in people, which makes them more pleasant to be around. What's not to like?
It's easy to make use of this saying, too, particularly in contexts surrounding the fluttery, gooey happiness that seems to spill over from people deep in the throes of a sweet romance:
Nick looked up from a much-creased, well-read letter and smiled at the sour-faced official hurrying down the corridor, who paused just long enough to offer a small, genuine smile of his own.
Bernard did a double-take. "Did fusty old Berringer just crack a smile at young Sayre?" he whispered to Merrick.
Merrick smiled as Nick walked past, beaming, and Bernard felt his own lips curve in answer. He'd rarely seen such a cheery grin.
"He did," Merrick said once Nick had moved along. "Sayre's been sickeningly happy since he got that letter from his girl, and you know what they say: all the world loves a lover. I suppose that's true, even for codgers like Berringer."
Since both of these sayings reference fairly universal feelings, you shouldn't have trouble fitting them into a lot of different kinds of stories: get-together, fluff, or even gen or angsty fics (you might not use them for the main characters if you're going for the angst, though I suppose it would depend on the source of the angst). Whether you use them or not, just keep on writing. All of fandom loves a good fic, after all, and when you write, the world—or fandom, anyway—reads with you.
"All the world loves a lover." Phrases.org.uk
"Laugh and the world laughs with you." The American Heritage Dictionary of Idioms, Christine Ammer. Houghton Mifflin, 2003.
"Laugh and the world laughs with you." Random House Dictionary of America's Popular Proverbs and Sayings, Gregory Titelman. Random House, 2000.