Why do terms for black people constantly become offensive after a few years?

I was looking at a website that lists the 2010 Census questionnaire and for race it has a box for "African-American/Black/Negr0". Hell, Y!A won't even let me type negr0 correctly, but it's legit with the Census. From Frederick Douglas to Booker T. Washington, blacks in the old days called themselves negr0s without a problem. If you look back on speeches by Malcolm X in the 60s he refered to himself as an Afro-American. Jesse Jackson did the same thing during his presidential campaigns during the 80s. Before that "colored" was acceptable, but now it's not. Why do these terms, like colored, negr0, afro-american, become acceptable for a while, then after a few years they are considered racist and new terms are invented? It seems bizarre, considering many of the terms, like afro-american, were picked out by blacks because they sounded better than the previous names that they were called. What other race has people speaking on their behalf and changing what they are called every decade? Heck, now I hear a push for them to be called black americans, rather than african americans. I can't keep up with this!