Rather than reinvented a wooden wheel, I can endorse the explanation for Erik Erickson's theory in Wikipedia. The short version is that Erickson proposed the people progressed through eight stages of development (unless something happens to stunt that progression), from Trust vs. Mistrust in infancy to Generative vs, Stagnation in one's elderly years. Each stage presents challenges as presented by the names of the stage. Many of the stages in early childhood involve developing skills and resolving questions concerning individuation, becoming a unique person who is confident, has self-esteem, and a set of values.
If a person fails to resolve a question at a particular stage, they continue through life but keep revisiting the stage they failed to resolve until they do resolve it. Failing to resolve one stage also prevents complete resolution at future stages. The manner in which stages are resolved affects a person's personality and how they resolved them can be seen in their personality characteristics. As with most theories, some truth can be derived from Erickson, or at least ways to explain certai phenomena.
My favorite aspect of Erisckson's theory, besides his book, Childhood and Society, was that he proposed that when we have found a method for solving a problem, we had a tendency to continually revisit it when we encountered similar problems. Erickson was a neo-Freudian, which means his theories were meant for personality development, but as my graduate professor were fond of showing, many of these theories were similar to cognitive theories of development, such as Piaget's, although Piaget's theories did not extend beyong adolescence and young adulthood.