For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
Hydrogen and oxygen and very reactive, which is exactly why they are so necessary for survival. Our bodies function off of chemical reactions, it makes sense to power that off of the most reactive elements it can easily find.