Contemporary speakers of American English are used to hearing the imperative Enjoy! uttered by waiters and waitresses upon presentation of the food ordered, but they are doubtless unaware of the usage’s provenience (Russian via Yiddish, as detailed in an earlier post [December 3, 2011]). Be that as it may, the lack of a complement––a direct object or a reflexive pronoun––after what is in standard English a transitive verb, is here to be explained as what might be called a functional ambiguity. Not specifying a complement syntactically allows BOTH the meaning of the direct object (it, i. e., the food) AND of the reflexive (yourself) to be implied despite their absence. This useful semantic portmanteau, of two meanings only by adumbration and not by the explicit presence of either, is what accounts for the spread of Enjoy!
By way of explanation from the structural perspective provided by markedness theory and its semeiotic understanding, the absolute (= intransitive) use of an otherwise reflexive verb to denote a state can generally be seen as an instance of iconicity: the reflexive-less form diagrammatizes the nonspecific (broadly defined, unmarked) meaning of the verb, whereas the form with the reflexive pronoun diagrammatizes a specific (narrowly defined, marked) meaning. Hence the change in the syntactic properties of enjoy that allows for its absolute use is just a garden-variety case of synchrony being the (cumulative) result of a teleological process.