Why do you think Janie decides to run off with Joe Starks?
The narrator tells us why Janie choose to run off with Jody. He represents "change and chance", a distinct contrast to her boring and predictable life with Logan.
As a young girl, Janie dreamed of a life filled with love and romance. She thought that marriage would naturally bring those 2 things into her life. However, her life with Logan has proved tedious and she feels there is something missing in her future. But although she is attracted to Joe, she is still unsure about him when they first meet. In fact, she says that he doesn't remind her of the flowers and blossoming of her youth, which is what she imagines her husband would always bring to mind. But at the moment he is better in her thoughts than Logan, so she begins seeing him. A week later, she runs away with him, although only after asking Logan what he would do if she left. She still has concerns for him, & doesn't just want to run away completely. But in the end, she leaves with Jody, for better or for worse. Of course, her life with him turns out to be much different than she expected, but it's another step on her path to finding her true self.
Jody tells Janie that he doesn't want to make a dog out of her, but a wife; currently at the time Janie's being used as a farm animal by Logan Killicks. It's natural that she, as a unique butterfly, would want to be set free. (You can use that last sentence.)
Basically, she wants to escape from her current life.