Emotions are chemical consequences

 Because of our large brains, human beings are capable of thinking about our problems reliving past events, or even forecasting future worst case situations and thus turning on the cascade of stress chemicals by thought alone. 

The body does not know the difference between the original event that created the stress response and the memory of the event. By repeatedly recalling the past experience, you can unintentionally anchor your brain and chemicals to relive the past. 

Emotions are the chemical consequences (or feedback) of past experience. 

When the fight or flight nervous system is switched on and stays on because of chronic stress, the body utilizes all its energy reserves to deal with the constant threat it perceives from the outer environment. Therefore, the body has no energy left in its inner environment for growth and repair, compromising the immune system.