Michael Reeves in his book Delighting in the Trinity sharing Richard of St. Victor’s (1150s) thoughts on the Trinity:

Richard argued that if God were just one person, he could not be intrinsically loving, since for all eternity he would have had nobody to love.

If there were two persons, he went on, God might be loving, but in an excluding, ungenerous way. After all, when two persons love each other, they can be so infatuated with each other they simply ignore everyone else – and a God like that would be very far from good news.

But when the love between two persons is happy, healthy, and secure, they rejoice to share it. Just so it is with God, said Richard. Being perfectly loving, from all eternity the Father and the Son have delighted to share their love and joy with and through the Holy Spirit.

It is not then, that God becomes sharing, being triune, God is a sharing God, a God who loves to include. Indeed, that is why God will go on to create. His love is not for keeping but for spreading.