At the root of it, love (which is a character of God’s being) requires more than one person. Kelly refers to Richard of St. Victor’s comment on this:
He saw that it was because of the nature of love. For love to exist, more than one Person is required so that the love may be shared. ‘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.’
(This seems intuitive and aligns with the John 17 vision of the relationship within the Godhead. It does make me think about John Piper’s consistent emphasis on God’s Passion for his own glory, which seems self-evident in scripture and which he argues is appropriate since the highest being that can be loved is God, and it makes sense for God to seek his own glory as the highest good. This is good and right, although I think the trinitarian character of that love and glory seeking provides a much richer vision. Not that I think John Piper would deny such, but it often reads in some of his texts in a monadic sense.)
That such a interpersonal relationship is inherent to God (quoting Staniloae):
If God needed to relate to something outside himself, this would imply that he lacked something distinct from himself. Divie relations must take place in God himself, although between distinct ‘I’s’, so that the relation and hence the love may be real.
Later, discussing the development of the concept of Person as expounded by Richard of St. Victor, Kelly cites T.F. Torrance regarding how our concept of Person is derivative of the theological concept:
It was from a theological understanding of God’s personal and personalizing self-communication, creating personal reciprocity between us and himself, that the Christian concept of the person arose, which is applicable in a creaturely way to persons in relation to one another, but which reflects the transcendent way in which the three divine Persons are interrelated in the Holy Trinity.
Without the Christian doctrine of the Trinity, there would be no Western concept of personhood.
References
- Kelly, Systematic Theology - Volume 1, p. 274-275
- Kelly, Systematic Theology - Volume 1, p.495