Children are made up of their experiences and early relationships are pivotal to their personality and character development. As such, it is important to understand these early experiences and relationships in order to make sense of why a child might be behaving in a certain way, reacting to some things in a particular manner, seeming very sensitive, having trouble managing his/her emotions, struggling with peer relationships, experiencing anxiety in school, etc.
Sometimes things happen in a child's life. The child may experience a loss (through death, divorce, etc.), rapid change (e.g., premature birth of a sibling, unexpected move or assimilation into a new community or culture, etc.), trauma, physical health problems requiring repeated medical intervention and separation from parents or physical abuse and neglect. The child may have been witness to parental discord or have a caregiver struggling with personal issues (e.g., alcoholism, depression, health problems, etc.). In some cases, there hasn't been a significant harmful event in a child's life; instead there's a mismatch between what the parent is able to give, emotionally and psychologically, and what that particular child needs. Even the most well intentioned parents cannot always control or predict how varied circumstances will come together and have an impact on their child's developing sense of self and, ultimately, the child's behaviors and relationships.
What parents can do, however, is recognize when their child is struggling with something and be open to seeking help. Parents shouldn't feel they have to go it alone. With therapy, children have an opportunity to process and work through their feelings in a safe, consistent environment without concern that they might be worrying their parents and parents have a chance to better understand their children and themselves in relation to their children.