The Relationship Between Will and Phänomena

Leibniz is operating on this Aristotelian conception of substance in which substances have to be self-active. So that's a definite motivation. But I think he's operating more on the accepting the human argument against causality. Causality can tell us why something moves at a particular time but it can't tell us why it moves at all, why there is movement. The unity of the will grounds these interactions between representations. That does sound right because that's just like the inside of us.