This incident involving David and Karen trying to chase Nic in their cars is a reinforcement, as David says, of the lack of control that they have in their lives. As in past encounters with Nic, even when they see him, they have no way of stopping him from walking out of the house or out of a rehab center—there is no way to make his choices for him. And yet, even with this knowledge, they are still desperately trying to maintain some control over him.