It is part of the worldview of A Course in Miracles that we are all One, and this One is God. Ideas (and we are all at base an idea) leave not their Source, and so we are created creatures, extensions of God Himself. Our mistake in wanting to be apart and separate can never satisfy, because we are meant both to have and to be everything. Our egos would want a little separate treasure, apart from what our brother has, but all this false belief can ever give us is isolation, loneliness, and fear. And God has protected Himself and us by seeing that this separation happens only in illusion, in dreams. And thus it does not happen in Reality at all.
This explanation can only be understood metaphorically in this world, because we are not yet back Home in God. But miracles can and do happen without a fuller understanding. We can be healed of our guilt over separation (and the illusory effects that it has brought) without sacrifice and without pain. All we need are open hands to accept the miracle.
Specifically, what we have to give up to be free of pain is fear. Jesus sums up how in one concise sentence, “Being wholly without attack, it [love] cannot be afraid.” (T-19.IV.A.10:7) Without attack, there is no fuel with the pain that it brings. The pain may, and frequently does, visit ourselves or others in the form of sickness. This, Jesus explains, is the faulty problem solving mechanism meant to illustrate the frailty of the ego’s home, the body.
In these few sentences is a summation of what Jesus tells us about the origin of pain. Is it understandable? Not at its introduction, because once ego defenses immediately activate to suggest a thousand reasons why this simple answer cannot be the whole of it. Give the ideas a while to prove their validity.