As always, context is everything. "A" here is the character Carol Connelly (Helen Hunt), and "B" is the character Melvin Udall (Jack Nicholson) in the film As Good As It Gets. Udall is an obsessive compulsive whose mental problems have stunted him emotionally, making it impossible for him to relate to other people.
When Carol asks Melvin whether her accepting his help obligates her, she's being incredulous. In normal human interaction, people often offer their help to others freely. But Melvin, not having normal emotional capacity, not only thinks that Carol's accepting his help obligates her to him, he can't think of any other possibility.