In the Course of Miracles it says:
To heal is to make happy. I have told you before 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 to 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 the influence whole-heartedly. But joy calls forth an integrated willingness to share in it and thus promotes the mind’s natural impose to respond as one.