They generally feel safe and secure in themselves and in their relationships. Their minds are not cluttered by hurtful things that people have done or said to them. Children get over things quickly. They are good forgivers and forgetters. They do not expend energy brooding or conjuring up ways to get back at people who have wronged them.
Adults tend to let things stick in their craw.
Tensions in relationships do not resolve easily. They may spend emotional energy trying to figure out secondary messages, what people meant by their actions and words. They are also occupied with what might be, or what might have been. They do not let things go like children do. As a result, personal buoyancy may be slowed and emotional resiliency weakened.