When people say that #blacklivesmatter they aren’t saying that all lives don’t matter. They’re simply pointing out that there is a significant issue that light needs to be shed upon.
If your first reaction to someone saying “black lives matter” is to say “well all lives matter” you’re clearly missing the point. Instead you’re taking away from people’s voices which are pointing out the injustice people face, and instead making it about yourself.
To me its very similar to when feminists are talking about the injustices carried out by men and people say “well not ALL men.” Instead of looking at the issue people are talking about you’re turning it around on yourself and devaluing the movement.
It frustrates me because to me I see this as selfish. When a kid falls down and gets hurt they get comforting words and help from others right? Well I see the people who are crying “not ALL men” or “ALL lives matter” as the jealous child standing in the background wondering why they didn’t get the attention.