I posed the following question to a friend of mine, who is currently in law school. From a practical standpoint I got some good answers from her and from a theoretical standpoint I found some great explanations from a few books I'm reading. Before I post a summary of the answers I received, take a shot at how you would answer the question, because it's really not that hard or complex to grasp the intuition behind it. Check back in a few days for an update, but feel free to leave your answer in the comments section!
The classical (read: Aristotelian) notion of contract (and the one applied in early U.S. history of the 17th and 18th centuries) has equity at the centerpoint of consideration. If person A entered into what was seen as an unequal (in value) contract with person B, person A was justified to sue for the difference. After the Revolution and as we moved into the 19th century, the freedom of the parties to enter or leave the contract became more and more important. It was less of a concern whether A entered into an unequal contract with B, and more of a concern whether A was forced into performance of the contract explicitly by B.
What drove the courts to change their understanding of contract from a primary consideration of equity to one of will?
Stay tuned for the answer...