Topics: Note to Self - WNYC
A new feature of The Sims 3 is attraction, which is available with the latest patch regardless of whether you have Seasons installed. It's a small mechanic, but it can give your single Sims a romantic target in case you have no other plans.
When there is attraction between two Sims, each gets a buff if they're in the same room together. Also, all Romantic-type social interactions between the two have a slightly higher chance of succeeeding, and the relationship score between the two will climb slightly faster. The bonus stacks with anything else going on with your Sim, so a high Charisma skill or several of the Lifetime Rewards will increase it even faster.
Attraction has the side-effect of motivating the townies to ask your Sims out on dates somewhat frequently. Other Sims don't respect your vows, so you may get a lot of calls asking your Sims on a date even if they're married. There is no way to turn it off (unlike opportunities), so it might get a little annoying if your Sims have particularly active social lives.