A wise engineer once said that all systems, regardless of composition, do one of three things: blow up, oscillate, or stay about the same. Once you understand feedback, you know why this must be. If a system blows up, it is governed by positive feedback. If it stays about the same, negative feedback is on the job. If it oscillates, either negative or positive feedback can be in charge. You have to look more closely at the details.
Feedback doesn't explain everything, but it explains a great many things. Let us now turn to how and why these three forms of behaviour: stability, oscillation, and runaway growth, are literally embodied in human beings.