"All lives matter" is divisive because "black lives matter" is a statement that black lives are largely devalued and forgotten in a system of institutionalized racism. "Black lives matter" leaves off the implied "too": "black lives matter, too."
> "WTF is the impulse behind changing #BlackLivesMatter to #AllLivesMatter. Do you crash strangers' funerals shouting I TOO HAVE FELT LOSS" - @arthur_affect on Twitter
> "WTF is the impulse behind changing #BlackLivesMatter to #AllLivesMatter. Do you crash strangers' funerals shouting I TOO HAVE FELT LOSS" - @arthur_affect on Twitter