Racism is the normal and natural condition of the human mind. It is a by-product of learning, and trying to avoid being "racist" goes against the grain, intellectually and emotionally. Most people who claim not to be racist are just kidding themselves. To a certain extent it is unavoidable.
Racism is born out of limited contacts with people who are "different." Most people live in segregated neighborhoods, states, countries, and even continents. We spend our lives, by and large, with people who look like us, regardless of whether we are white, black, Indian, Mexican, or whatever.
Since our contacts with people who are "different" are limited, our brain seeks to learn about them by making generalizations based on the views that we have. It's like trying to figure out a jigsaw puzzle; we see bits and pieces and the brain automatically tries to fill in what we don't see to make a coherent picture.
Under some circumstances people are allowed - or forced - to interact regularly with people who are "different," and it is interesting to observe (or experience) what the mind does with the conflict between one's mind-created perception of "different" people (a "stereotype") and the people of that race or group whom we come to know personally - who are different from the stereotype. I can remember as a kid, each mini-neighborhood in my area of Pittsburgh had its own single-race make-up, but there were a few individuals of the other race who for one reason or another happened to be a part of a "different" group. Occasionally, when talking to the black kids in my group we would refer to "*******" from another neighborhood. THESE guys weren't "*******," because we knew them, but the other kids from a couple blocks away were definitely "*******."
If you don't live under circumstances where you either voluntarily or by force are interacting with people who are different, then ALL YOU KNOW about them is what you see on television, in film, at sporting events, and in the streets. And suffice it to say that for most White Folks like this, the majority of the Blacks we see are criminals on the news and in fiction, the ass-holes who make fools of themselves any time they make a good play in basketball or football, and the ones who are "acting out" in the streets.
And when those pieces of the "jigsaw puzzle" are the only ones you have, your brain doesn't paint a very flattering picture of black people in general. You assume they are all degenerates, for lack of a better word.
Conversely, if you are a black kid who only sees the white people on television, the white policemen who patrol the neighborhood, and people riding by in cars that your family can't afford, you very well might conclude that white people are (1) rich, (2) self-absorbed, and (3) hateful. Which is accurate, but not helpful.
Like I said above, I believe that the people who claim most vociferously that they are not racist are generally either lying or kidding themselves.