Jesus teaches that the model prayer should begin with a genuine faithful heart of reverence toward God [to whom we are praying]. This is in stark contrast to the methods of the hypocrites and the pagan idol worshipers who have no genuine reverence for their god to whom they pray.
The genuine prayer of the faithful heart is one that acknowledges at the outset of the prayer who is being addressed. Since God the Father is the holy and almighty God the supplicant (the one praying) is to address HIM as He reveals himself to be. "Our Father" is a term of familiarity and trust. It is a relational title that contradicts the false prayers of the pagan idol worshipers and hypocritical performance prayer.
God is due honor. His name is holy. His name is to be hallowed. All prayer must begin with genuine humility and reverence even to the familiar loving God who reveals himself as Father.