Why do people get angry if someone says all lives matter?

Depends on context.

Because saying it in response to black lives matter is an attempt to make us change the topic.

It is not like people who say all lives matter are out there putting much effort into the cause. It's sentence against activism and not for it.

You have to fight for your own rights, not tell others not to fight for theirs.

Yes, all lives matter. I think everyone agrees on this. But this movement is about something more specific, the rights and equality of black people. To simply throw the rest of civilization into the mix is to dilute the message. It's like showing up at a birth and declaring all babies are beautiful or saying at a 9/11 memorial that all buildings are important. It misses the point.

All lives matter, but not all lives are treated like that. White people aren't getting unjustifiably killed and beaten by the police. White people don't risk losing their lives simply for being who they are. You could think of the slogan as, "Black Lives Matter, So Let's Treat Them Like They Do."

(This was written by a white person, so take it for what it's worth)

Because they want to tell us what , when and how to speak and think

Not certain its always anger. Of course all lives matter and everyone should be able to feel that way.

They want to be superior, if all lives matter then they wont be.

people delete my post if I mention this