In the nineteenth century, it was considered entirely too familiar and improper for a single man to address a single woman by her first name. If he addressed her by her first name, it implied that they were engaged.

Let's use the Emma Thompson version of the film Sense and Sensibility as an example since more people are likely to have seen that than read the Jane Austen novel. At the end of the film, Hugh Grant playing Mr. Farras addresses Miss Dashwood (Emma Thompson) by her first name of Elinor. She begins to weep. The subtle nuance often lost on the audience is that Elinor suddenly knows by the use of her first name that he intends to propose. He doesn't get to finish because it's already there and implied by the simple use of her first name.

Additionally, the use of a person's first name was considered so personal that many in the nineteenth century continued to address each other by their last names around other people even after they were married. An example of this (to use another film that most people have likely seen) was evident in Gone with the Wind in the scene where Scarlett's mother comes home and tells "Mr. O'Hara" that he must dismiss the overseer and he responds by calling her Mrs. O'Hara. That was common in upper-class families. Only when they were alone or in private letters would they have addressed each other by their first names or private nicknames. Lower classes were less formal about the use of first names after people were married.