I personally hated hearing this growing up. "Be nice" is one of the most harmful things you can tell a girl to be. It makes us question our judgement and think we are wrong for feeling our feelings. I also read an article in Elle where Brie Larson said she ad-libbed the line about "be nice" It was Larson who came up with the line in ROOM, screaming at her mother (played by Joan Allen): "I'm sorry I'm not nice anymore!" In the movie she says something like (Im paraphrasing from memory), "if I hadn't heard your voice in the back of my head to "be nice" maybe I wouldn't have stopped for him and I wouldn't have been kidnapped and locked up for 7 years!" That line stuck out to me because I totally heard "be nice" and hearing that command made me so angry as a girl. Now I'm glad to know I'm not the only one that feels that way about the words "be nice."