The Sims 3/Relationships

Relationships
There are three basic concepts to relationships in the Sims 3. Long term relationships, short context relationships and commodities.
 * Long Term Relationship: Describes the way two Sims view each other at a given moment. The long term relationship starts when 2 Sims first socialize with one another. The long term relationship is measured by the green bar below the Sim you are talking to in the upper left hand corner of the screen during a conversation. When a relationship is positive the bar fills up with green. If the relationship isn't doing too good, the green bar will empty out as the conversation progresses and can even sink into the red.

Visitor Privileges
Visitor Privileges are things a Sim can do on another Sim's lot. At 1 Sims can do basic things like eat food that is offered to them, clean dishes and use the bathroom. As the number increases there are more visitor privileges available.

Long Term Relationships

 * Short Term Context: Short Term Context is what a Sim thinks about another Sim during a conversation. Short Term Context is in the upper left hand corner of the screen when two Sims are having a conversation. Example "John thinks Paul is being sociable." or "Mary thinks Peter is being friendly." Here is a list of the short term contexts : Akward, Amusing, Creepy, Drab, Dull, Friendly, Frightening, Funny, Hilarious, Hot, Impolite, Insufferably Tedious, Insulting, Odd, OK, Seductive, Steamed, Unforgivably Rude, Very Akward, Very Friendly, Very Scary.
 * Relationship Decay: Relationships between Sims decay every day. Sims need to keep in touch to avoid realtionships becoming too low.

Friendship
To build up good relationships with Sims, call them on the phone and invite them over. Using alot of "Friendly" social options and socials directly related to positive traits a Sim has (such as the Ambitious trait) are a good way to boost a friendship.
 * Some Sims want to have long term relationships with other Sims. (Good friends, best friends, get engaged; etc.) Having friends results in opportunities and positive moodlets. Positive moodlets are the key to earning Liftime Rewards.
 * Social is a need unless your Sim is a loner.
 * Chatting with co-workers outside of work can lead to promotions.

Romance
Two Sims must have a high enough relationship in order to be romantic with each other. The best way to build a romance is to work on a friendship and when it is high enough start using Romantic socials. With enough romantic and friendly socials, your Sims will soon have the options to go steady, woo-hoo, get engaged, and eventually get married.