> The statement "Black Lives Matter" offends me, because it makes the assumption that because I'm not black, I don't think black lives matter, and therefore have to be told that they do.

There is no such assumption embodied in the statement, and to assume there is requires the extreme vanity of assuming that the slogan must be a message about you. Rather, the message is that the manner in which black people are treated overall in society demonstrates that society at large acts as if black lives do not matter, and this needs to be corrected. It does not assume that this behavior is either universal among or exclusive to non-blacks (BLM expressly concerns itself with ways in which the black community, including other "black liberation" movements, marginalize certain segments of the black community.)