Why did Jesus refer to the Father as “my God”?

Perhaps to head off reasoning such as this (Panikkar, The Trinity and the religious experience of man;: Icon-person-mystery ): “God is only God for the creature and with reference to it. God is not ‘God’ for himself. The idea of worship is inherent within the concept of God. It would be an absurdity to say that God can worship himself. It is the incarnate Son alone who calls his Father God, and in the great theophanies of the Old Testament Jahweh always reveals himself as the God of those to whom he is manifesting himself. He never says ‘I am my God!’ but ‘I am your God.’ Without us and apart from our relationship to him God would not be ‘God.’ God is not God by himself; he is so only for and hence through the creature.”

Yes, an absurdity, but a Trinitarian absurdity, fighting as always against Nestorian reasonableness.