Why do we have to act like grown ups?

Before Sharon arrived, I told Oleg how her husband doesn't like being around us when we're together. 

Me: "He thinks we act like children."

Sharon arrived, and it was some time before she remembered she should shoot off a text to let her husband know she'd gotten to us safely. He responded immediately with information on what he and the kids were planning for dinner and entertainment, and then he ended the text with, "be safe and watch your back."

Sharon: "What the hell does that mean? Watch your back?"