The Father willed His own hypostasis (Athanasius the Great, Third discourse against the Arians). A single God because a single Father (Athanasius the Great).
“The ultimate and the foremost point of reference for the existence of God is not the Essence, it is the Father . . . you cannot proceed from the essence of God in order to subsequently reach the Father. You start from the Father, and, because He claims to be a Person and not an essence, He bears the characteristic of not being ‘only;’ whereas essence can be understood as something on its own, and essence is of course one . . . The one Essence is not a precedent, and it is not the ‘cause’ of God’s being. Precedent is the Father, as the agent . . . [for] the only way that God can exist, is as a Trinity; and this is attributed to the Father – a person – and not an essence. Therefore the essence – albeit important and necessary – does not have that primary significance of causing God to exist. That which causes God to exist, is the person of the Father . . . [because] the divine essence does not automatically and naturally produce the Triadic Life as a natural consequence. It produces it via the Father – via a Person” (Zizioulas, J. 2005. On God: Basic principles of Patristic teaching).