Stress in humans and animals is a biophysiological process that is coordinated by the brain via the autonomic nervous system. In reaction to sensory information from any source (emotional, physical, mental, or environmental pressure) that is interpreted as danger or threat to the organism, the parasympathetic nervous system is activated.
Biological perception of threat or danger activates the fight, flight, or freeze response and associated behaviors to re-establish homeostasis through a counterbalance of biochemical interactions within the body. You are either at biological peace or you are not.
The brain is in charge of monitoring the environment for sensory information that indicates danger or threat to the organism. This is carried out at the cellular level through biochemistry, not at the human intellectual level, which is confusing to try to understand.
In the same way sharks can sense the subtle electromagnetic emanations from inoperable debris floating in the water after a shipwreck, humans and animals communicate sensory information with each other through the interaction of their individual electromagnetic emissions at a level below human awareness.
When the threat response system is activated, there will be hyperarousal of the hypothalamus-pituitary-adrenal axis, which includes physiological symptoms of the stress response. Just because some do not believe they are under stress at, an intellectual level, the body actually tells the truth of the matter.
Individuals can be toxic to each other and can activate the stress response system in each other. If you experience a racing heart, panic, nausea, anxiety, gastrointestinal distress or any of the other stress symptoms, identify you trigger, which may be someone whom you love or is in your family. Just staying away from someone does not address the problems leading to activation of the stress response system in each other, which happens at an unconscious level.
If you are not dealing directly with the behaviors that are upsetting your homeostasis, you will not resolve your problems. You have to deal directly with changing your interpersonal interactions so that your body is at peace whenever you are around the individuals who provoke danger or threat to your nervous system when you interact together.