When I think about things in the past I've done or experienced that cause worry, I can often feel similar to the time it happened - anxiety, fear, remorse. It may not be to the same extent when the event occurred, but it is still a reaction. Physiologically, the hormones that are released are lets say..."not good". They may even be harmful healthwise.
At the time, the response by the body might serve a purpose. However, re-living it and experiencing the same response serves no purpose.
This phenomenon is similar to worrying about future events. There is a difference between planning for the future and predicting it.
For me, when I am in the past or the future, I can't be present in the now - in today. What's more important is that I experience unhelpful thoughts and therefore reactions by my body. It affects anxiety and distracts me from the current day AND the feelings of peace, joy, contentment and safety I experience when I'm truly staying in the present. My body can produce other hormones or chemicals....better ones....that are helpful to my overall well being.
I am trying to be kind to myself today - mentally and therefore physically.