By the end of toddlerhood, important human lessons must be learned: that people can be trusted; that warmth from others is possible; that verbalizing feelings leads to greater comfort than behaving impulsively; and that continuity of relationships is the rule and not the exception.
These convictions and such basic ego capacities as the ability to distinguish reality from fantasy, to modulate mood, to control thinking and behavior, and to create an accurate sense of who one is ("sense of self") are critical if a child is to feel confidence and realize joy in living.
This is why the severer psychological disorders which derive from the earliest years are devastating, making many of these sufferers incapable of living an independent adult existence and achieving satisfying relationships. Leading to continuing frustration and despair, and even suicide.