For one, a reaction may occur within the space of seconds. Since it is usually immediate, it is often without any thought or deliberation, and may, therefore, not be the optimal way in which an individual would have liked or preferred to handle a situation. Reactions are, however, normal and expected. Problems arise when the immediacy of a reaction - as opposed to a response - causes interpersonal difficulties for the reacting individual. Reactions are often emotionally charged and, therefore, tend to be problematic, especially when they may be associated with anger. Those things that we all sometimes say that we wish we could take back are probably reactions, rather than responses.
Typically, responses are the outcome of thoughtfulness, reflection, consideration for relevant factors, and are often formulated and well presented. Responses are not usually those things that we "shoot from the hip," but offer with care, tolerance for differences, and respect for those with whom we interact.