Given that a purely actual reality (a reality who exists through the power of its own essence, whose very nature is "to be") that we call "God" can be deduced by natural, metaphysical observation, a profound, mysterious implication bluntly presents itself in the form of a question. Why, if God is pure act, does He in fact continuously create and sustain all existent things? Why is there Creation at all from the divine perspective? Even if our understanding of God's divine action is reduced simply to the sustaining of the motion of all things and not of their existence, as Aristotle believed, the question of God's motive remains essentially unchanged. Why does He sustain the motion of all things at all? Aristotle would have us believe that since God is the highest metaphysical "reality", He can only think about Himself as the highest reality. Now there is, of course, truth to this theological doctrine, as will be explained, but as will be argued the reconciliation of this divine "selfishness" and the divinely originated continued sustaining of the continued existence/motion of all things finds its solution only in Trinitarian Christian theology which understands God as an essentially communal, omni-benevolent Creator/Sustainer.
Let us first reflect on the theological implications of continued creation/sustenance. If God is indeed Actus Purus (pure actuality), then He must be ultimately metaphysically simple due to the fact that there can be no potency in pure act, by definition. To be complex in the metaphysical sense of the word implies an inter-connected and inter-dependent network of actuated potencies (potentials) within the nature of that complex entity. But the Sustainer of conditioned reality must be ultimately unconditioned and, thus, have no unactuated potency (a potential to be something that has not yet been actualized) that can only be actuated under the necessary condition of its actuality coming to be in act by something already in act. Therefore, God, who is Actus Purus, must be metaphysically simple, devoid of any and all unactuated potencies. Since God's existence is deduced from the natural, metaphysical observation of contingent things, God's relationship to these things must reveal an essential component of His character, but to consistently maintain the doctrine of divine simplicity argued above, this component must be in analogically ascertained, synonymous harmony with the other components of the Divine Essence. Therefore, whatever is the nature of the relationship between God and Creation must reveal the nature of the supremely simple essence of God.
What, then, is the nature of this relationship? If God is pure act, then He is necessarily devoid of all potency. Therefore, God's activity cannot in principle be to the actualization of a potency He wants to be actualized for his own telos (end). Such a state of affairs would blatantly contradict the essence of Actus Purus. God's Activity, therefore, must be to the actualization of something "apart" from His self, for that something's own sake, by necessity of the nature of Actus Purus (pure act). This lies at the very heart of love. Love, in its purest sense, can conceptually be reduced to the willing of the good of the other for the sake of the other. Since the world exists, and God's existence is rationally unavoidable from observation of the world, this synthesis is, by logical extension, also rationally unavoidable, that "God is love, and he who abides in love abides in God, and God abides in him." ( 1 John 4:16).
Now that the difficulty of reconciling God's essence as pure actuality and the existence of the world has been resolved, it is now necessary to treat the other difficulty in reconciling the veracity of Aristotle's theological doctrine with the love of God demonstrated above. As mentioned, this reconciliation can only be made in terms of Christian theology. Christian theology holds that God is essentially a unified relationship, a community of love existing as the essence of God. God the Father is the lover, Christ the Son is the beloved, and the Holy Spirit is the shared love between the two persons, generating a third person, all unified in one, ultimately simple essence whom we call "God". Though difficult it is to philosophically explain the possibility of this relationship in relation to the unity of God, it is not difficult to turn to this as the solution to the problem mentioned above. God, as love (that love must be part of the analogically ascertained unified divine essence has been shown above), loves Himself through Trinitarian dynamics as the purest, simplest love conceivable. Love, however, since it inherently points beyond itself towards the good of the other, by nature radiates beyond its source to something other than itself. Hence is why creation/sustenance is a necessary activity of God's essentially loving will. God, as pure love itself (implied by the doctrine of divine simplicity), exists essentially as a Trinitarian relationship since love is inherently relational as a concept by itself, yet necessarily radiates beyond Himself as a self-sustaining fountain of this love, sustaining His own essence as love through love yet gratuitously radiating that love beyond the source which is Himself as the extended yet necessary activity of that love. Since He is Love itself, sustained by His own essence of love through love, He must still radiate His love beyond the source which is Himself in order to ultimately fulfill the essence of love, which is to will the good of the other for the sake of the other.
This is how the reconciliation of Aristotelian divine "selfishness" and the gratuitous sustaining of the world on part of the divine is made through Christian Trinitarian theology. It does not fully make sense of the Trinity since the Trinity is an intrinsic mystery, but it does demonstrate its necessity and explanatory power once the existence of God as pure act has been rationally demonstrated.