If you know a bit about embryology, then you do understand that before any system is formed in the foetus, the first organ to take shape is the heart, it is the heart(Qalb) that starts our journey of life and therefore the first time we skip a heart-beat is the time when God sows the essence of life into our bodies.
So, if it is the heart that further helped the devolopment of the 🧠 brain or organ systems which give us the senses of pain, sight, taste, hearing etc, then how can the systems that are secondary to the heart or develop much after the heart has begun its journey decide whether the heart should continue its beat(journey) or not? And if we look at modern science, even if one looses his or her ability to see or hear or think or rationalize, the heart can still continue to beat and never develops a cancer(ever hear of a heart tumor?).
In conclusion, our heart was the first to detect our worldly being and is the last to let go of our material being...and when we give our physical powers of sense,touch,smell,sight and pain etc the ability to stop our heart from beating(euthanasia/assisted suicide), we are then, in effect, letting an inferior system decide the fate of a superior one.