What makes an animal Ectothermic/Endothermic in regards to Physiology?

Yes I know what it means to be cold or warm blooded. What is going on inside out bodies that makes us regulate a steady temperature. Why is it that a humans temperature cannot fluctuate much while a reptiles body can? Cold blooded creatures don't need to eat as much as warm blooded ones and they can regulate their metabolism. It seems to me that it is wasteful to use so much energy regulating body temp. What is even occurring inside an endotherms body that regulates this constant temperature?