Analysis sets going or accentuates a play of forces within the self between two groups of factors with contrasting interests. The interest of the one group is to maintain unchanged the illusions and the safety afforded by the neurotic structure; that of the other group is to gain a measure of inner freedom and strength through overthrowing the neurotic structure. It is for this reason that analysis … is not primarily a process of detached intellectual research. The intellect is an opportunist, at the service of whatever interest carries the greatest weight at the time. The forces that oppose liberation and strive to maintain the status quo are challenged by every insight that is capable of jeopardizing the neurotic structure, and when thus challenged they attempt to block progress in one way or another. They appear as "resistances" to the analytical work, a term appropriately used by Freud to denote everything that hampers this work from within.
Resistance is by no means produced only by the analytical situation. Unless we live under exceptional conditions life itself is [a] great a challenge to the neurotic structure. … A person's secret claims on life are bound to be frequently frustrated because of their absolute and rigid character. Others do not share his illusions about himself, and will hurt him by questioning or disregarding them. Inroads upon his elaborate but precarious safety measures are unavoidable. These challenges may have a constructive influence, but also he may react to them first with anxiety and anger, one or the other prevailing, and then with a reinforcement of the neurotic tendencies. He becomes still more withdrawn, more dominating, more dependent, as the case may be.
— Karen Horney in Self Analysis (1942)