To be blunt:

1. All lives do matter.
2. Nobody needs to be reminded of that.

We do, however, need to remind some of the less intellectually developed people that black lives, do indeed, matter. Which they do, they always have, and always will. While this is a subset of everyone's life mattering, it does not and should not imply that the lives of non-black persons do not matter, it should only serve as a reminder that black lives do matter.

The problem is that people get so fragile about being left out that excluding them from a movement because nobody ever needed reminding that their lives "matter", causes them to get offended that someone else is getting special attention that they do not deserve, and have no right to impose themselves upon. Saying "all lives matter" in response to the BLM movement is, in my mind, a form of what aboutisms which is toxic and diminishes the point of BLM. It's ignorant and short sighted.