Frankenstein's monster relates to Milton's Satan specifically. In chapter 15, the monster learns to read by finding books at the DeLacy's home. He takes several books, but says that "Paradise Lost excited different and far deeper emotions" than the others did. The Romantics, specifically Lord Byron, interpreted Milton's Satan as the hero of Paradise Lost because he fit the Romantic ideal of "courage, and majesty, and firm and patient opposition to omnipotent force," (Percy Shelley, Preface to Prometheus Unbound). When looking at Paradise Lost, the creature says that "Many times I considered Satan as the fitter emblem of my condition" than Adam. It's true that in Milton, Satan's character has all the best speeches; however, Satan was not really the hero in Milton's eyes since Milton held strong Christian beliefs. Making Satan appealing served Milton in showing how sin and evil can be tempting and appealing to our own sinful nature as humans. One of the key marks of Milton's Satan is his ability to speak beautifully, which is perhaps the quality which the monster learns the best. From that point on, the monster's language is complex and beautiful. Because the monster sees himself more in Satan than in Adam, he emulates the way Satan speaks and carries himself from that point onward. 

The monster's—and the Romantics' by extension—incorrect reading of Milton displays one of Mary Shelley's overwhelming points across the entire novel: learning must be guided. That theme is shown from the very beginning with Victor's creating in isolation and dabbling in alchemy without the supervision and guidance of a teacher.