Friendship

Friendship is common relationship state in all PC, console, and handheld versions of The Sims. When two Sims' (daily) relationships are over 50, the Sims become friends.

Becoming friends with another Sim allows greater social interactions, which are less likely to be rejected by the receiving Sim. This means that gaining a higher status relationship (Best Friends, Crush, etc.) is easier to achieve.

The Sims
In The Sims, Sims became friends with each other when their relationship meter went over 50/100.

Originally there was only one meter in which to measure friendship, but with the release of the Hot Date expansion pack Sims now have two relation meters. One Daily meter, which measured a Sims' short term relationship status, this meter decayed rather quickly if a Sim didn't keep in contact with their friend, and a long term relationship meter. The long term meter measured the Sims' friendship over the long term and would take a while to both build up and decay.

The Sims 2
The Sims 2 largely continues the Hot Date formula for friendship, but keeping friendships becomes far easier as their relationships decay at a far slower rate than before. Different relationship levels are also given a different description if the player hovers over a Sim in the relationship panel.

The Sims 2 also introduces the Best Friend friendship level, which allows Sims to become best friends after their long term relationship meter is high enough. This unlocks more social interactions for Sims.

The Sims 3
In The Sims 3 friendship and relationships in general were given a slight overhaul. The long term relationship meter and the short term bar are now merged in to one. Because of this the progression from Acquaintance to Friend to Best Friend and the new BFF state are done linearly with the one bar. Friendships are also no longer represented numerically. Friendships do still decay but the decay seems to be slower. It is difficult to tell since there is no longer a number to look at, just a bar that fills up.

If a relationship decays below friendship, the other Sim now becomes a "distant friend", so they aren't lost as a friend. If the friendship decays more they become acquaintances again.

The Sims 4
In The Sims 4, sims become friends when the friendship level right 35%. Once two sims becomes friends, they will text and call the other sim, and invite them to hang out on a lot together. Unlike The Sims 3, there is no friendship decay.

The Sims 4 also introduces the Good Friend friendship level. The good friendship level is when the friendship level is 60% or higher. This allows sims to connect more and ask to become best friends if their relationship is high enough. This also unlocks more interactions.