The Arcane Nature of Life
Anyone who has watched a creature die knows that there is a moment when life passes. The shell of crude matter after death is no different than it was before, save for the missing spark. The nature of life is not contained in the physical form, but rather in an energy not found in other applications on this Earth.
The most effective means for the creation of arcane effects is to use arcane energy for the purpose. The most potent source of arcane energy in our realm is life. The ancients knew this as well. The idea of sacrificing an animal, or indeed, a person, is centered around the idea that there is energy within a living creature that can be released with its death. Those who practice the arcane arts explain that the energy of life can be more precisely metered — blood, pain, and other forms of suffering all can be tapped for arcane energies.
Highly skilled practitioners are more efficient with their conversion of energies. Subtle changes in technique can greatly change the energy requirements. Even with that advantage, every arcane action taps a bit of waters of life. With prudent use and long periods of rest, a practitioner may recover nearly all the life that was spent, but a fraction always disappears forever. There is a reason why the popular depictions of witches and warlocks show a gaunt and haggard soul.