I believe the be-nice rule shouldn't merely apply to what you say, but to everything that crosses your mind as well.
Yesterday, I realized how hard this can be, particularly when it's about the people that are close to your heart. I realized how I've left things unsaid myself.
I use that phrase because it reminds me of an old blogpost by someone I met long back in college: The things left unsaid —Alaric Moras. I hope you read that post, because it's really good.
It's hard to speak out and be truly honest sometimes, and it's way way harder when it's someone you care a lot about. How ironic!