How they build Friendships/Romances, and what you can do to speed up that process. I'll provide tips for learning a Sim's Traits and getting the Friendship bar to move faster in the positive direction as well as useful Socials for making Friends.
For this one, I'll focus on the positive relationships in The Sims 4, although you can certainly make enemies. It's just much easier to make an enemy than a friend - just be mean. If you want to declare an enemy, the option will be under mean when the bar drops low enough. When the green bar progresses to the right, the relationship between your selected Sim and the target is building. When it reaches the following percentages, you are at that Relationship Level:
- Acquaintances - 0-39%
- Friend - 40%-59%
- Good Friend - 60%+
The ability to be Best Friends is unlocked once you hit 80% with another Sim. You must ask the Sim if they want to be Best Friends, but they'll usually accept if they like your Sim.
As Sims gain standing with one another, more options open up when visiting in their home. Sims who are in a romantic relationship and achieve Boyfriend/Girlfriend status can freely sleep, clean, and cook meals in the other Sim's home without being rude. There is a lot more to what interactions are available, however...