How do sims become best friends?

[beardeux]

I have a sim who has 2 friends and the friendship bars are full.  They are only good friends.  Is that the high end now?

[ClayMask]

Your Sim can become BFFs with one of them, but only one at a time.  You do this by having your Sim ask to become best friends underr the friendly category.  Otherwise, the highest is good friends.

[beardeux]

Ok, thanks, but I've not seen that option either.  I'll look for it more diligently.

[beardeux]

Hi ClayMask,  I did finally have that option show up.  I think I must have seen it with my first sim because he had a best friend who is now his soulmate and girlfriend as well as best friend.  I don't remember it though.  So when I tried to make him my next sims BFF, there was no option.
So I thought if I go went back to him, there might be an option to break the BFF, or at least by talking with my new sim he would get the option to change and make her the BFF, but that never appeared.  Is there a way to change the BFF status?  It seemed the only way to make that change was to break up the existing relationship and I don't want that.

[ClayMask]

It seemed the only way to make that change was to break up the existing relationship and I don't want that.

I haven't tried breaking the BFF status, but I've heard you have to get the relationship bar to nearly the bottom if they're both still alive in order to it, unfortunately.

[lindsaysoderberg]

I haven't tried breaking the BFF status, but I've heard you have to get the relationship bar to nearly the bottom if they're both still alive in order to it, unfortunately.

I have broken it many, many times (for the Friend of the World Aspiration). Considering how long it takes to build the relationship up to BFF status, it is a little painful at first. 

Luckily, sims are quick to forgive, and you can build a relationship back up to friendly again right away if you want.

One thing to note: some unplayed sims already have BFFs, and you have no way of knowing it. For example, none of my sims were able to become BFF with Mortimor or Bella Goth. I assume that they are each other's BFFs. For that reason, when trying to make a BFF, I avoid married sims.

Good luck!