Polyvagal theory is based on the notion that our physiological and emotional states determine how we operate in the world. People who have very sensitive nervous systems, such as those who suffer from trauma, anxiety, and hypersensitivity can have a very difficult time feeling attuned and focused in social interactions.
This ability to feel safe with others is mediated by the vagus nerve which is part of the autonomic nervous system. Stephen Porges named this the Social Engagement System (SES) which refers to this part of the nervous system that picks up non-conscious cues about whether a person or situation is safe or not. It is always operating, which is great in the sense that it keeps us alive. On the other hand, it can be exhausting for those whose nervous systems are highly sensitive and are constantly hypervigilant to potential threats, even when they are completely safe.