There is a difference between the created good and the creative good. The saying “goodness is diffusive of itself” as a natural, involuntary quality refers to the created good, and this quality was infused by the creative good who wills it to be so. In other words, God being the creative good itself, does not need to share of Himself, but He freely chooses to do so unconditionally.
By the same token, our human nature is good, but because we have free will, which participates in the creative good, we have the ability to will good and also to will evil. Without our choice, our goodness does not diffuse of itself. But if we choose to do good, the good we produce would sooner or later be evident as it is diffusive of itself.