What causes me to shut up?
My wife, when she gives me "the look."
My boss, ditto.
Pretty much any overheated, idiotic absolutist "debate" that isn't worth my time.
Extreme views based on the echo chamber (e.g. anyone who swallows everything Fox News spouts without question; anyone who swallows everything MSNBC spouts without question).
Any discussion demonizing other points of view. Being an adversary is healthy. Being an enemy is not. There's a difference... In this country the parties used to be adversaries. Now they're enemies. That's a very bad thing.
Any discussion that interprets "equality" as "special treatment." I DON'T CARE whether your plumbing is on the inside, what Pantone shade your skin is, what language you speak, which disabilities you've got, or what floats your boat in the bedroom. Since everyone else gets celebrated for their own particular thang, I'm campaigning for Middle-Aged, Middle-Class, Straight White Guy Rights. And I want my own MAMCSWG Pride Parade. So there.
I also firmly believe that religion and politics are not fit topics for polite conversation. I have my views. They probably aren't yours. So let's just move along... nothing to see here.