Because God is triune, he has eternally been personal and relational in his own being, in full independence from his creation. God has never had any unmet needs, "nor is he served by human hands, as though he needed anything, since he himself gives to all mankind life and breath and everything" (Acts 17:25). Personhood becomes real only within realized relationships, and the reality of relationship can only exist where one has something or someone that is not oneself to relate to; if, then, God had not been plural in himself he could not have been a personal, relational God till he had begun creating, and thus would have been dependent on creation for his own personhood, which is a notion as nonsensical as it is unscriptural. Between the persons of the Trinity, there has always existed total relational harmony and expression; God is, from this standpoint, a perfect society in himself. Apart from the plurality in the Trinity, either God's eternal independence of the created order or his eternally relational personal existence would have to be denied.
From my ESV Study Bible.