The most prominent advocate of a contractarian view of trusts, Professor John H. Langbein, points to the deficiencies of the early law courts as to why the law of trusts was not initially treated as a contract:
Recall Maitland’s insight that ‘a trust generally has its origin in something that we cannot but call an agreement.’ Maitland was asking why the early law did not immediately assimilate the trust to the law of contract. His answer has never been doubted. The common law of contract was too primitive to do the job: “If … in the 14th century our law of contract had taken its modern form, I think that the courts of law would have been compelled to say ‘yes, there is an agreement; therefore it is a legal enforceable contract …'
Despite the actual historical origin of the law of trusts and the imprint that this history made on that law, Professor Langbein sees the enforcement of trusts as more properly a part of the law of contract: “In truth, the trust is a deal, a bargain about how the estate assets are to be managed and distributed.” Thus, the role of fiduciary duty is nothing special, has no moral footing, and is merely a contract obligation:
[D]espite decades of pulpit-thumping rhetoric about the sanctity of fiduciary obligations, fiduciary duties in trust law are unambiguously contractarian. The rules of trust fiduciary law mean to capture the likely understanding of the parties to the trust deal, which is why both the duty of loyalty and the duty of prudence yield to more particularized intentions that the parties may choose to express or imply in their trust deal. I depict the default regime of trust law as a type of standardized contract, and I point to some instances in which the contractarian perspective should improve outcomes in trust law.
The law governing fiduciary duty, however, came by its “pulpit-thumping” roots honestly and those roots serve the “institutional integrity” of the trust and its progeny.
 Langbein, “Contractarian,” supra note 6, at 634.  Id., at 62.  Id., at 629. The leap from a contract theory of trust law to the view that fiduciary duties are merely default rules is recent: “Although the argument that trusts are a species of contract has existed for at least a century, the precise characterization of fiduciary duties as mere ‘default rules’ crystallized only in the past fifteen years.” Leslie, “Trusting Trustees,” supra note 13, at 76.  Although decrying the pulpit-thumping rhetoric that surrounds fiduciary duty, Professor Langbein states that his contractarian analysis is not meant “to fold the law of trusts into the law of contract” because “the trust has an institutional integrity and convenience that fully justifies its jurisprudence.” Langbein, “Contractarian,” supra note 6, at 630. Part of that integrity, presumably, arises from the success that has been achieved by the common law in defining the fiduciary duty owed by trustees in moralistic terms.