Although it doesn't surface until the final two lines, architecture forms the poem's final image. The soul is the metaphorical architect in question, although a high-paying, prestigious, and widely-admired position this is not. The body makes it clear that the natural order, as opposed to whatever is civilized or built up, is superior. Green trees are happier than wooden fences and bodies without souls are happier than bodies with souls. Why? Because of the change in capacity, that's why. A tree is a tree but a fence can keep people out, block paths, and function as a symbol of hostility. In the same way, a body with a soul becomes capable of a whole range of new things, some good and a lot bad. By "building" a body with consciousness and a conscience, the soul sets it up to commit sin.
Lines 41-42: In this metaphor, the soul is a carpenter or architect, fashioning the body into a receptacle for bad thoughts, wicked desires, and sins galore.
Lines 43-44: Just as architects cut down trees and transform them into new, less-natural shapes, so the soul takes a body and warps it into a conscious, moral human being. And in a wicked world, with temptations around every corner, that is a seriously bad thing.