The Law of Conservation of Energy states that energy cannot be created or destroyed, just transferred and transformed.
This means many things, not the least of which is that energy itself, across the entirety of the universe, is finite. There’s only so much that can exist. There’s only so much capacity.
Energy comes in many forms in this universe, just as it does in the universe of your mind and body. We live lives in denial of this inconvenient truth on virtually every scale of human existence. We burn through resources in all their iterations with little thought for what is lost.