Every time #BlackLivesMatter is at the forefront of the conversation, (white) people want to bring up the counterargument that all lives matter, which is a true statement. All lives do, in fact, matter, but that's not what the statement or the hashtags has come to mean.
I have only ever seen people say that all lives matter while arguing against the Black Lives Matter movement. When media attention was on the immigrant children being locked in cages at the border, I didn't see anyone saying that all lives matter in defense of the children and migrants. After mass shootings, I don't see anyone saying that all lives matter in favor of gun control. When indigenous peoples were fighting against an oil pipeline that would poison their sacred water supply, I didn't see anyone say that all lives matter in an attempt to protect the #NoDAPL protesters.
The only time people seem to bring up #AllLivesMatter is in going against #BlackLivesMatter. It's brought up to discredit the movement and take attention away from the issue at hand: Black people's lives are treated as disposable and countless institutions are meant to reinforce that.
By saying that all lives matter, you're not exactly saying that Black lives don't matter, but you are saying that attention does not need to be given to the injustices that Black people face every single day. You're not exactly saying that you don't believe that Black lives matter, but you are saying that you either don't believe that racism and discrimination still exist or that you don't care enough about it to speak out against it.
By saying that all lives matter, you are not saying that all lives matter. You are saying that you are ignorant to the way this world treats Black people and/or too privileged to actually care about Black lives. You are taking attention away from the lives that are systemically treated as if they don't matter.
I'm not going to give metaphors to explain this point because Black people should not have to be compared to animals, forests, diseases, or houses that are on fire in order for non-black people to understand that black lives matter. I'm simply going to leave it at this: yes, all lives do matter, but that's not what #AllLivesMatter is about. It's about going against #BlackLives Matter. It's about refusing to acknowledge that Black lives are seen as disposable. It's about taking the focus away from the word Black and from the real issues at hand.
#AllLivesMatter is about privilege and ignorance, so think carefully about yourself and your opposition to the word Black next time you say it.