Trauma

Trauma is not what happened to you, but what has been left behind in your somatic memory. If you have felt in danger, your autonomic nervous system remembers, and your ('rational'), accessible memory may not. So that's why we can feel a physical response to a situation that doesnt necessarily fit... It might feel like an overreaction. 

Our nervous systems are constantly scanning for cues of safety or danger at all times under the level of our conscious awareness. If we assess there are more cues of safety than danger, we can feel connection with ourselves and others. If our nervous system detects more cues of danger than safety, we are disconnected and can be feeling a fight, flight or freeze response. 

When we have been exposed to many incidents of danger growing up, our nervous systems are more tuned to cues of danger than safety. Its hard for us to feel connected. We have trouble in relationships and we often self soothe with substances or other maladptive coping mechanisms.