Singles are always relieved to be single again, even if that's all they've ever been.
Don't ever imply that a single is unhappy with their current relationship status or they will be forced to scream defensive, Oprah-reminiscent statements at you until your ears bleed and you truly believe they're "independent, happy and healthy", much like 2+2=5 and that we have always been at war with Eurasia.
Single people require you to listen as they list the reasons their previous or potential partners are all wrong for them and they are waiting for the "real deal".
If you have found the "real deal", it is not helpful for you to describe it and how you achieved it to them or to try to aid them in finding it for themselves. Like shoe-tying, finding true love is something people must do on their own.
Singles do not expect your pity. Rather, they expect many opportunities to meet friends of your partner and "socialise" in a "fun, harmless" way. This is your obligation as a happily coupled-up person, much like paying taxes when in a high-income bracket.
To remain single is always a choice. Always.
Don't take a single's word for it when they tell you that "nothing happened" with that cute person before they then smile coyly and walk away. It is necessary for the survival of your relationship with said single person that you neither believe nor disbelieve them. You must then take on the task of badgering them constantly for "details" about their encounter very seriously. These heavily choreographed and scripted conversations about non-existent flings are bread and butter to bored singles.
"Meeting people" really is as hard as singles say it is. The fact that you've "met someone" should always be put down to luck because if it's based on merit, timing or maturity then singles really have something to worry about.
Being single is something people "love" but don't ask them to use the l-word in any other context; it may force them to admit that there are relationship statuses they appreciate more than their current one.
Nothing sets a single off like the l-word, Valentine's Day and news about weddings. Especially the last one. If you're getting married and your friend/sibling/co-worker is feeling a bit fragile already, sometimes it's best to just not tell the single at all. Walk yourself down the aisle. It might encourage some empathy.
Singles understand relationships, they just don't want one. That's why they're still openly pining for their previous paramour a year after the split. They're happy alone.
People who are single avoid relationships because it would take time away from their career and plans. Things that are too much fun to cut back on.
If a single tells you they've been really busy with other things lately, smile and nod. They need to think so. Really.
Single people take the time to enjoy life in many ways. One of the best ways to do this is talking (or writing blogs) about how great it is to be single.
Tune in next time for Stuff Sarcastic People Like.