From the book A Course In Miracles…
To heal is to make happy. I told you once to think how many opportunities you have to gladden yourselves, and how many you have refused. This is exactly the same as telling you that you have refused to heal yourselves.
The light that belongs in you is the light of joy. Radiance is not associated with sorrow. Depression is often contagious, but although it may affect those who come in contact with it, they do not yield to its influence wholeheartedly. But joy calls for an integrated willingness to share in it, and thus promotes the mind’s natural impulse to respond as one.
Those who attempt to heal without being wholly joyous themselves call forth different kinds of responses at the same time, and thus deprive others of the joy of responding wholeheartedly. To be wholehearted, you must be happy. If fear and love cannot coexist, and if it is impossible to be wholly fearful and remain alive, then the only possible whole state is that of love. There is no difference between love and joy. Therefore, the only possible whole state is the wholly joyous.
To heal, or to make joyous, is therefore the same as to integrate and make one. That is why it makes no difference to what part or by what part of the Sonship the healing is done. Every part benefits, and benefits equally. You are being blessed by every beneficent thought of any of your brothers anywhere. You should want to bless them in return, out of gratitude.
You do not have to know them individually, or they you. The light of joy is so strong that it radiates throughout the Sonship and returns thanks to the Father for radiating His joy upon it. Only God’s own holy children are worthy to be channels of his beautiful joy, because only they are beautiful enough to hold it by sharing it. It is impossible for a Child of God to love his neighbor except as himself. That is why the healer’s prayer is, “Let me know this brother as I know myself.”
A Course In Miracles, Urtext, p.123
(in the edited version: Introduction to Chapter 5)