Healthy hair comes from a healthy hair follicle. That's it. The follicle is naturally designed to grow thick, healthy hair, but like every other organ in your body, it needs a normal, healthy supply of blood flow to do its job.
Blood flow is the delivery system that carries oxygen, nutrients, and vitamins to the follicle. When blood supply gets restricted, the follicle weakens. As it weakens, it produces thinner and thinner hair, until eventually it can't produce hair at all. That's what we call "hair loss," but it's really just the end result of a much longer process.
You can actually predict the health of your hair by looking at the diameter of each strand. Thick hair means a healthy, well-fed follicle. Thinning strands mean a follicle that's been slowly starved of blood supply. If your hair is thinner today than it was in your 20s, that's the story your follicles are telling you.
Nobody teaches us how to take care of our hair the same way we're taught to take care of our bodies. But the principle is the same. Focus on the health of the follicle, and the hair will take care of itself. You don't have to spur new growth. You just have to restore the environment the follicle needs to do what it was already built to do.