Yes, all lives should matter, I think we could agree on that.
But the point of black lives matter as a movement is to raise awareness that all lives don't matter to our systems and institutions. All lives really do matter to me, too, but I also acknowledge that our systems do not reflect that truth, our neighbors and friends and family telling their truths are enough evidence. So I support black lives matter. so we can hope for a world where all lives actually matter.