Human beings are hugely complex biological systems, and by far the most complex of any system known to man, organic or inorganic. During the process of development it is easy for disorders to develop.
There are some biological events that make disorders more likely to occur such as lack of oxygen at birth, chromosomal birth defects, poor diet, physical injury to the head or nervous system, and ingestion of toxic substances to name a few, but by far the greatest factor in most psychological disorders stems simply from the physiological development from conception to adulthood. As behaviour develops in the growing infant so too does the structure of the brain as a result of the thought patterns that the individual uses in carrying out their chosen behavioural patterns. If the structure of the brain and its connnections with the body develop poorly, then the chances of psychological disorder increase. Brain structure and physiological connectivity do not develop as a result of genetic makeup or diet or any biological factor, but as a result of behaviour. Behaviour might be modeled off other people, it might be learnt through rote practice, or it might be the result of trial and error, experimentation, curiousity, play or any other means either deliberate or unplanned. Most behaviour is a function of social interaction. This is especially true in a society that interacts through global trade, telecommunications and the internet.
The biggest factor in determining psychological disorder is social interaction (or lack of), the most influential social environment is the immediate family, and the period of greatest influence is the first five years of life.
The distinctions between characteristics we inherit genetically from our parents and those we acquire through environmental exposure are blurred. Even the nine months of gestation in the mother's womb offers an ample window for the acquisition of copied behaviour. The mother passes on clear messages to the growing foetus on her emotional makeup by the way she deals with situations and the growing baby can also get some idea of other people by the way the mother interacts with them and how she feels as a result. In addition many parts of the complex machinery of her biological makeup are the result of her own functional development (rather than genetics), such as her level of fitness, health, the way she moves and what she is capable of doing, the environments she exposes herself and her baby to etc. The environmental factors that influence, either directly through the term of the pregnancy, or indirectly through the functional development of the mother, the growing foetus are considerable. After the baby is born those influences increase even more and there are a great many studies that indicate that the first five years of a child's life are the most important in determining the quality of development.
Outside of the family, the power of social influences on the individual are usually weaker. Next is the extended family: aunts, uncles, grand-parents then close friends of the family, school, peer groups, pop culture, community leaders and so on. Obviously the extent of influence varies from individual to individual and if the family is a weak influence then influence will be gained from elsewhere.