The human body is full of defence mechanisms. Billions of years of evolution has led to a species that is designed to survive a huge amount. I suspect this is largely due to the individuals need to pass on their genes(something they cant do if they are dead)
You are in a car accident, adrenaline hits the vascular system causing vessels to constrict which limits bleeding. The heart beats faster, breathing increases in depth and rate allowing oxygen to get to the muscles. This allows you to get away from the danger and survive long enough to reach hospital. This is not such an uncommon scenario. Unfortunately when you get to hospital the first question you are asked is usually 'where does it hurt?' and the adrenaline is numbing the more significantly injured areas. The risk comes when the adrenaline is no longer being secreted and patients go from stood up talking calmly to dead on the floor in the space of seconds.
You lose a loved one in this same crash. Your mind decides how to process the loss. It could repress it, it could compartmentalise the feelings or it could allow you to grieve and release them. The immediate best option is chosen to allow the individual to survive as strongly as possible.