So if you have
low cholesterol, normal
blood pressure, healthy diet, no smoking, exercising, then that basically leaves genetics which is the bucket where the unknowns are thrown into. It may not be your reason, but many people don't even realise they are stressed. It becomes a part of every day life for many, and they don't see the impact it has on the body until it's too late. I had a few classes in relaxation, to overcome my stress.