[testbot123]

"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

[smitty1e]

I feel as though this discussion has more to do with the control over the discussion than with the content of either phrase.

[eesmith]

Well, yes? That is, the phrase "all lives matter" seems like it's often used as a way to take control away from the discussion about largely devalued and forgotten black lives.

If you want an explanation better than "Oh well, you know", you might start with the criticism section of its Wikipedia entry, at https://en.wikipedia.org/wiki/All_Lives_Matter#Criticism .