[1auralynn]

I think it might be something about the shared mental state and bonding... maybe releases serotonin and/or oxytocin - kind of a "monkey grooming" type thing.

I noticed this one time when I was in college: I had come home on a break and couldn't relax, I was super stressed out and felt generally bad. I went over my best friend's house down the street, bitched to her about some small stuff that had annoyed me that day, she mirrored me and I INSTANTLY relaxed. I remember it 15 years later because it was such a marked effect.