I thought BLM was stupid and I was in the "all lives matter" camp until someone told me an excellent analogy that changed my mind.
Say you work in a factory, with the same skills, same experience, doing the same job as everyone else, and you find out you're getting paid $5/hr less than everyone else.
So you go to your boss and say "I think I deserve equal wages". And your boss says "I think everyone deserves equal wages, don't you?"
And you might say "Well, yes, that's basically my point...but this conversation is about me, because I'm the one not getting equal wages here."
Your boss tries to make a case that you're being selfish by making the conversation about just you deserving equal wages, but in reality your boss is completely missing the point about what the real problem is.
That's how BLM folks feel when someone says "All lives matter!"