From a Meditation by Richard Rohr – Palm Sunday, March 24, 2013:

Richard of Saint Victor, who died in 1173, once said, “For God to be good, God can be one.  For God to be loving, God has to be two because love is always a relationship. ”  But his breakthrough was saying that “For God to be supreme joy and happiness, God has to be three.”  Lovers do not know full happiness until they both delight in the same thing.

When I was first becoming “known,” people wanted to get close to me and be my friend or have a special relationship with me.  I asked myself how I would choose between all these friends and I realized that the people I really found joy in were not always people who loved me — nearly as much as people who loved what I loved.  That helped me understand what I think Richard of St. Victor was trying to teach.  The Holy Spirit is the shared love of the Father and Son / Mother and Daughter / Husband and Wife, and shared love is always happiness and joy.  The Holy Spirit is whatever the Father and Son / Mother and Daughter / Husband and Wife are in love with together;  the Holy Spirit is that excitement and happiness and joy!

Adapted from The Shape of God: Deepening the Mystery of the Trinity by Fr. Richard Rohr of The Center for Action and Contemplation