One theory is that the inability to remember certain memories is the brain repressing the information and thus making it inaccessible to the conscious mind. Freud is most connected with this psychodynamic theory, often called motivated forgetting.
He believed that highly emotive memories, principally painful, are actively forced out of the conscious mind to the unconscious mind often as a defense mechanism, where they are inaccessible but continue to have influence over decisions. Naturally, this theory has been difficult to test, but several studies such as those by Levinger and Clark (1961, as cited in Gross 1996) and Eysenck (1974a, 1974d, as cited in Eysenck 1977) have shown that high levels of emotional excitement can be adverse to information retrieval. In addition, such disorders such as psychogenic amnesia, posttraumatic stress disorder, and event-specific amnesia can be explained in terms of memory repression, though perhaps less in a Freudian sense than by a view that the memory is able to make some of its contents unavailable as a coping mechanism.
However, there are many explanations that also fit such circumstances of forgetting, such as retrieval failures, distortion of one’s own views, and false memories. False recognition was investigated in Clancy, Schacter, McNally, and Pitman’s study (2000), in which it was found that women who had reported recovered repressed memories were more prone to false recognition than control subjects, which may suggest that memories are subject to distortion in some people more than others.
Emotions:
To behave functionally according to evolutionary standards, the mind's many subprograms need to be orchestrated so that their joint product at any given time is functionally coordinated, rather than cacophonous and self-defeating. This coordination is accomplished by a set of superordinate programs - the emotions. They are adaptations that have arisen in response to the adaptive problem of mechanism orchestration.
I found this example on fear:
Consider the following example. The ancestrally recurrent situation is being alone at night and a situation-detector circuit perceives cues that indicate the possible presence of a human or animal predator. The emotion mode is a fear of being stalked.
(In this conceptualization of emotion, there might be several distinct emotion modes that are lumped together under the folk category "fear", but that are computationally and empirically distinguishable by the different constellation of programs each entrains.) When the situation detector signals that one has entered the situation "possible stalking and ambush", the following kinds of mental programs are entrained or modified:
(1) There are shifts in perception and attention: You may suddenly hear with far greater clarity sounds that bear on the hypothesis that you are being stalked, but that ordinarily you would not perceive or attend to, such as creaks or rustling. Are the creaks footsteps? Is the rustling caused by something moving stealthily through the bushes? Signal detection thresholds shift: Less evidence is required before you respond as if there were a threat, and more true positives will be perceived at the cost of a higher rate of false alarms.
(2) Goals and motivational weightings change: Safety becomes a far higher priority. Other goals and the computational systems that subserve them are deactivated: You are no longer hungry; you cease to think about how to charm a potential mate; practicing a new skill no longer seems rewarding. Your planning focus narrows to the present: worries about yesterday and tomorrow temporarily vanish. Hunger, thirst, and pain are suppressed.
(3) Information-gathering programs are redirected: Where is my baby? Where are others who can protect me? Is there somewhere I can go where I can see and hear what is going on better?