Why should or shouldn't we allow the human body to take its natural course?
I am assuming that your question is: "why can a human intervention improve health?". Let me know if I misunderstood your question.

Why can a human intervention improve health?

Let's first avoid going into the details of your example. It is quite obvious that human intervention can often improve health in a way that your body alone cannot. To me, an intuitive way to classify the reasons why human intervention are important to improve health into two categories.


The body does not always react in an adaptive manner.


Example: Anaphylaxis is a serious and sudden allergic reaction that may cause death. An allergic reaction is what is happening when your immune system recognize a chemical as a infectious element while it is not. The reaction of the body is not adaptive and taking medication such as an anti-histaminic can force the body to stop this "stupid" reaction.
Note: There are reasons why the body cannot be always perfect but it is a bit long to make an overview here. There is stochasticity in the developmental processes, there is genetic variation for example due to always occurring deleterious mutations, there is also an arms race between parasites and host. This arm race leads parasites to take advantage of normal host physiological pathways. There are tons of other reasons that relate to the stochasticity and to the physic and physiological constraints of evolutionary processes.

The body sometimes cannot (physical constraint) produce the action that is required to be performed to improve health.


Example:  If you have an important wound, then a human-made compression can by far improve your chance of the survival. The body is not able by itself to simulate this external compression to prevent blood to exit the body through the wound.



You might easily find semantic issues with these categories though such as the question of cancer and its removal for example and some may argue against this categorization.

Dehydration and thirst

Your example would fall into the first category. It is not so uncommon that the body needs water while not causing thirst. Because after 50 years old, the sensation of thirst start to diminished, dehydration without the sensation of thirst is common among old people. Dehydration along with hyperthermia is a relatively important cause of death during hot weather in summer. It is especially true in old people as the sensation of thirst diminishes with age. Dehydration following diarrhea is also a common and important issue in old people sometimes leading to death.