Traditional Theism defines God as omnipotent, omniscient, and omnibenevolent. In order to be the supreme being, or the greatest conceivable being, God cannot be limited in power or knowledge and the conjunction of perfect power and perfect knowledge entails perfect goodness (in so far as God interacts with the universe).
However, once the three ‘o’s are established as necessary qualities of God, other metaphysical properties like eternality, immutability, and impassiblity also follow. It is the last of these that is of particular interest to Hartshorne.
If God is impassible, then it is impossible for God to love us in any meaningful way. And, if God does not love us, Hartshorne believes the very heart of religion is undermined. Therefore, he proposes rolling back these philosophical claims about God which push God further away from us. He argues we should re-conceive of God as limited, but in appropriate ways. For example, social relationships require mutual dependence (if I am absolutely independent, I have no need of others), although it need not be symmetrical dependence (God can still be greater than us, just not absolutely great).
More importantly, Hartshorne argues for Sympathetic Dependence: an interdependence rooted in sympathy between God and the creation.