It's not a closed relationship. The exchange of love that is God so overflows itself that creation occurs. Creation is God's love expressed outside of the Trinity. But creation doesn't just happen, spontaneously, as if by an accident. No, God's creative love was by intent.
Divine love is self-giving love.
The fullest expression of this self-giving, reciprocal and creative love is revealed in the life, death and resurrection of Jesus, the Paschal Mystery by which all creation is created anew. Sin cuts us off from love. Sin separates from others. Sin is the opposite of God's love. God's love is all-encompassing.