I'd like to explain why I think this happens by making a connection with Catholic sacramental theology. This is a point I've made before in relation to enchantment, but I'd like to show how it connects to ethical action as well.
Specifically, as physical creatures we can't relate to the spiritual world directly. We have to approach spirit through matter. You can't love spirit directly. You have to love spirit through matter.
Given, then, that God is a spirit, we can't love God directly. Oh, we can try, but we'll find that our "love" struggles to find traction and purchase in this unseen "spiritual" space. Trying to love God directly Gnosticizes our love, shifts love away from tangible, physical expressions toward something vague and abstract. A "spiritual" love that doesn't have a material object or outlet tends to reduce to mere sentiment.
I think that is what Jesus was getting at with the Greatest Commandments, that these aren't two different loves, but that we are to love God through loving our neighbors.
I know it sounds radical to say that you can't love God directly. But 1 John 4.20 makes the point well:
"If someone says, 'I love God,' but hates a Christian brother or sister, that person is a liar; for if we don't love people we can see, how can we love God, whom we cannot see?"
