"Association" in psychology refers to a connection between conceptual entities or mental states that results from the similarity between those states or their proximity in space or time. The idea stems from Plato and Aristotle, especially with regard to the succession of memories, and it was carried on by philosophers such as John Locke, David Hume, David Hartley, James Mill, John Stuart Mill, and Alexander Bain. It finds its place in modern psychology in such areas as conditioning and in neural network models of memory.
-wiki Association Psychology
Associations form between temperatures, auditory cues like days of the week and times of day, visual clues like sunshine, snow and rain, tastes and textures like pumpkin pie, birthday cake and sensations like sex, kissing, hugs, pats, hand shakes and being abused physically.
Disorders form as negative and positive associations with the wrong thing build in a persons life. When I was 12 my mother stopped talking to me. She did not require my father to honor the court ordered visitation rights. She may have sent cards a few at my birthday and Christmas but it felt shameful that such little effort was being made to parent. My paternal relatives encouraged a quasi-negative view of her. The pain of the constant negative events surrounding my birthday and then christmas caused me to develop negative associations with those days. It also caused a positive association with other people birthdays. I tried to celebrate them to a certain extent as I grew older. Exacerbated by the fact that in general I grew up poor and my grandparents struggled right before christmas and around my birthday financially due to thier kind of business. My grandfather passed away in mid winter and each year I remember his passing and am sad around that time as well. Its not really a disorder only a sadness at that which has happened in my past.
We may pass these associations on to our children like plants do in a epigenetic method called genetic memory.
In biology, memory is present if the state of a biological system depends on its history in addition to present conditions. If this memory is recorded in the genetic material and stably inherited through cell division (mitosis or meiosis), it is genetic memory.
-wiki Genetic Memory
Major depression sometimes happens when everything has a negative association. Some of those are rational others are not. They crowded out the positive ones and prevent the person from being able to see the good in life. Psychologist use some reconditioning of the feelings associated with some memories to retrain the brain not to subconsciously be so sad.
Bipolar disorder happens when the brain breaks trying to correct negative or positive excitement caused by memories or sensations. Venturing too far in the opposite direction.