Relationship

As soon as a character (Sim or pet) meets another they accumulate relationship points. The number of points two characters have for each other and their life stages determines how they can interact. The score will go up or down depending on whether interactions between the two are positive or negative, but neglecting a positive relationship will result in a gradual decline of the relationship score. However, a negative relationship score will gradually improve until it reaches zero.

The Sims
In the initial release of The Sims, there was a single relationship bar which ran from -100 to 100. The Sims: Hot Date turned this into the daily relationship bar, and added a lifetime relationship bar. This arrangement was carried forward into subsequent expansion packs, and into The Sims 2.

Two Sims would become friends if each one had a daily relationship of at least 50 toward the other. If they also had lifetime relationships of at least 50 toward one another, they would become best friends.

Once two Sims had any kind of relationship, the lifetime relationship between them would build as they spent time together on the same lot, even if they didn't interact. However, this increase in lifetime relationship would come at the expense of their daily relationship. Every so often, the daily relationship would drop by two points, and the lifetime relationship would increase by one point.

The Sims 2
The Sims 2 retains the daily relationship score and lifetime relationship score that were introduced in The Sims: Hot Date. In The Sims 2, the lifetime relationship score gradually approaches the daily relationship score, so over time, two characters' lifetime relationships will build up as long as they continue to interact. Sim relationships could also be raised or lowered with the use of cheats.

Pets develop relationships with Sims and other pets. Sims gain relationships with pets by doing positive interactions, such as praising, playing, or giving love. Negative relationship points are given when a Sim scolds or yells at a pet. When teaching a pet tricks, the relationship goes up and down, depending on how fast the pet learns, but in the end is higher.

Casual

 * Just Met: The two characters have met, but have not interacted enough to have a definite relationship. Daily relationship will be close to zero, though it may be positive or negative.


 * Acquaintance: The two characters are not friends, and the one whose relationships are being looked at does not consider the other an enemy. The relationship scores may be positive or negative.

Inimical

 * Enemy: Daily relationship of -15 or less.


 * Some versions of The Sims 2 also have Arch Enemy, but it appears that the PC version does not.

Amicable

 * Friend: Each of the two characters has a daily relationship of at least 50 toward the other.


 * Best Friend: If each of the two characters has a lifetime relationship of at least 50 toward the other, this relationship will be set when they have a positive interaction.


 * Best Friend Forever: "Best Friend Forever" was introduced in The Sims 2 FreeTime, and reportedly happens when two characters have been best friends for a certain amount of time.

Romantic

 * Crush: Daily relationship of 70 or higher.


 * Love: Lifetime relationship of 70 or higher.


 * These relationships will not automatically be lost when the relationship falls below the required level.

Pets' Levels
Cats Dogs
 * Mine: The cat claims ownership of the Sim or pet; similar to Best Friends
 * Master: The dog is loyal to the Sim or pet
 * Pack: The dog recognizes the Sim or pet as a member of his pack; similar to Friends.
 * Best Friend: The dog treats the Sim or another pet as a close friend.

The Sims 3
The relationships in The Sims 3 are slightly different from its predecessors. It does not measure how many "relationship points" a Sim has, and Sims have one relationship bar. Before two Sims meet, they are strangers, but they become acquaintances. The next level in an amicable relationship is friend, then good friend, then best friend forever. In an inimical relationship, the first level is "disliked", then enemy. In a romantic relationship the first level is "romantic interest". Then, if one Sim proposes to go steady with the other, boyfriend/girlfriend. Then again, if one Sim proposes to the other, it's fiance, then husband/wife.

Marriage
Sims require a high relationship to become married or joined, but the state of being married or joined is not lost autonomously if the relationship score declines.