Stephen W. Porges said this quote

Playing nice comes naturally when our neuroception detects safety and promotes physiological states that support social behavior. However, pro-social behavior will not occur when our neuroception misreads the environmental cues and triggers physiological states that support defensive strategies. After all, playing nice is not appropriate or adaptive behavior in dangerous or life-threatening situations. In these situations, humans - like other mammals - react with more primitive neurobiological defense systems. To create relationships, humans must subdue these defensive reactions to engage, attach, and form lasting social bonds. Humans have adaptive neurobehavioral systems for both pro-social and defensive behaviors.

~ Stephen W. Porges

A child's (or an adult's) nervous system may detect danger or a threat to life when the child enters a new environment or meets a strange person. Cognitively, there is no reason for them to be frightened. But often, even if they understand this, their bodies betray them. Sometimes this betrayal is private; only they are aware that their hearts are beating fast and contracting with such force that they start to sway. For others, the responses are more overt. They may tremble. Their faces may flush, or perspiration may pour from their hands and forehead. Still others may become pale and dizzy and feel precipitously faint.

~ Stephen W. Porges