C. S. Lewis:

All sorts of people are fond of repeating the Christian statement that ‘God is love.’ But they seem not to notice that the words ‘God is love’ have no real meaning unless God contains at least two Persons. Love is something that one person has for another person. If God was a single person, then before the world was made, He was not love. (Mere Christianity [revised and enlarged edition, HarperSanFrancisco, 2001], ch. 2; pp. 160-161.

Richard of St. Victor, a 12th century Scottish theologian ministering in France, put it like this:

One never says that someone properly possesses love if he only loves himself; for it to be true love, it must go out towards another. Consequently, where a plurality of persons is lacking, it is impossible for there to be love. (De Trinitate, III.2)