Dehumanization occurs primarily as a result of being part of a larger specialized system, with a sense of futility of being able to change the course of that system. A person who cannot face shame, guilt or pain, of what harm the system that he is part of is doing to others, either rationalizes it by perceiving his role as fragmented and refusing to acknowledge responsibility for what the system is doing, and what he is doing as a part of that system, or by viewing those hurt by the system as subhuman and somehow deserving the pain. The major example would be Nazi Germany, where many Germans were "just following orders" (as Adolf Eichmann said) and where the Jews and other minorities, as well as disabled people, were portrayed as less than human and not deserving to live or to be treated with freedom and dignity. Other genocides have also taken place in modern times, usually based on a similar rationale.
A part of dehumanization is blocking out one's emotional feelings of empathy toward others. Sometimes it would seem necessary to do so in order to help others, such as when a doctor performs surgery. In general though, by blocking out such feelings, one not only can become more cruel toward others, but also toward oneself; in that he becomes a more repressed person, less able to share feelings of love or concern for others. Furthermore, in a large world such as ours, it is natural for one to care little about mass tragedies happening elsewhere, yet much more concerned about the trials of one friend, or a pet. The "good German" attitude caused by dehumanization becomes a vicious cycle: as people see their roles as fragmented, they become apathetic and allow those in power to accumulate even more power.
There have been suggestions for minimizing dehumanization. For example, encourage people to exercise more power in the decision-making process. Another example, educate children toward rehumanization and against behavior that might lead to dehumanization. And encourage people to think more in terms of the future, including the long-range future, so they may respond to situations which might lead to dehumanization. In Christian terms, we can each begin with ourselves, acknowledge our guilt and responsibility for our own bad actions, acknowledge our responsibility as part of a larger system, and become less apathetic, with the hope that we can each see ourselves and others as fully human.