One of the most common responses to "Black Lives Matter" is "all lives matter." But that response misses the point, as this great cartoon from Kris Straub at Chainsawsuit demonstrates:
The point of Black Lives Matter isn't to suggest that black lives should be or are more important than all other lives, but instead that black people's lives are relatively undervalued in the US (and more likely to be ended by police), and the country needs to recognize that inequity to bring an end to it.
Reddit user GeekAesthete made this point in a thread explaining why the phrase "all lives matter" is offensive:
Imagine that you're sitting down to dinner with your family, and while everyone else gets a serving of the meal, you don't get any. So you say "I should get my fair share." And as a direct response to this, your dad corrects you, saying, "everyone should get their fair share." Now, that's a wonderful sentiment -- indeed, everyone should, and that was kinda your point in the first place: that you should be a part of everyone, and you should get your fair share also. However, dad's smart-ass comment just dismissed you and didn't solve the problem that you still haven't gotten any!
Straub's cartoon echoes this point: If a house is burning down, you're obviously going to focus on putting out the fire instead of watering a house that's just fine. In this analogy, black lives are the burning house, and everyone else is living much more comfortably in the house that isn't burning down. Clearly, one is a bigger problem.