Aquinas argues that we should love God more than ourselves, for example in the Summa Theologiae here (with reference to the angels and the natural love of God) and here (with reference to us and charity). Now, it seems that if you were to ask a simple Catholic the reason why we love God more than ourselves he would not talk about God being the common good of all creation or as the cause of happiness.
Loving God more than ourselves seems to be part of Sacred Tradition; is it explicit in Sacred Scripture? One could interpret "greatest commandment" (Matthew 22:36-40) as implying this order. It is only implicit in Luke 10:27 -- it must be read into it, from Tradition and perhaps from Matthew 22:37?
It seems that through the theological virtue of Faith we believe that God is more lovable than us or more worthy of love. If pressed for a reason we might talk about His goodness and His relationship to us as Creator to creature. He is the cause, we are the effects, we are subordinate to Him. He is also "greater" in perfection, "better" than us. It seems to me that Aquinas's argument with God as the universal good is a more sophisticated version of this line of reasoning. If asked to give an explanation of the order of charity as it pertains to God, I think many of the faithful could manage an explanation with the sort of amateur theology I've outlined above.