Singles bring what I call "single person energy" (SPE). Most are on the lookout for a mate, so they often put more care into their personal appearance than attached people do. They make an effort to talk to strangers because they don't have the crutch of a partner. And though it's certainly not true across the board (there are plenty of singles who stay at home all the time and couples who have adventures), those without mates often lead more racy lives that make for better anecdotes. When people pair-bond, they can lose interest in their social lives—as if all that partying and doing shots was nothing more than a strategy to find a mate.

When partners socialize separately, this unlocks their latent SPE. Like single people, they have to make more social effort because they can't use their partner as a crutch. They can also indulge in harmless flirting, and a little sexual tension usually improves a party. (Caution: Not everyone is comfortable letting his or her partner flirt, however innocently. A quarrel will decidedly not improve the party.)