Taking selfies with other sims gives a massive amount of friendship points.
No idea if this is a bug or not, also not sure if this is where I should post this (these forums are confusing) but using the "take photo with" action twice and taking 10 photos with a sim gets me alm...
I noticed this too recently. Like my Sims are friendly with another Sim then all of a sudden after taking a selfie I get the notification that they're good friends now.