one assumption is the childhood experiences, where Freud believed in five psychosexual stages of development in children, in which the libido is focused on an area of pleasure.
the five stages are oral, anal, phallic, latent and genital, and how the child develops in these stages will be reflected in their adult personality.
if a child is frustrated or overindulged during any of these stages, then fixations will occur, leading to problems in adult life.
for example, a fixation in the oral stage of development could lead to an eating disorder in adulthood, or an addiction such as alcohol or smoking.