When Janie is stuck in a loveless marriage with Logan Killicks, Joe Starks seems to be the man who will "rescue" her. Joe represents everything that Janie longed for before getting married, as well as everything that Logan could not provide: she meets him spontaneously, not in a pre-arranged fashion, and she is physically attracted to him. Joe sweeps Janie off her feet and seems to be the savior that she has been waiting for.
We are aware of Joe's ambition early on. He wishes play an important part in the creation of a town just for black people. Once he arrives in Eatonville, he uses his intellect to become mayor. He makes it clear that he has big plans for the new town and his position in it, saying that he is "buying in, and buying in big."
It quickly becomes clear that Joe's ambition puts a strain on his relationship with Janie. He puts his work as mayor before his wife, and their love weakens steadily over time. He uses Janie as a tool to gain influence, putting her in charge of a store that is the center of life in Eatonville, and at times shows her off as something of a trophy wife to underscore his power in the town.
The conflict of interest between Joe's ambition and his marriage with Janie is developed further during the incident with Bonner's mule. In an apparently selfless act, Joe buys the overworked mule presumably to make Janie happy. Instead, the act soon turns into a way for Joe to gain more support by showcasing his compassion. When the animal dies, he holds a mock funeral for it, but does not allow Janie to attend the funeral because it would be improper for someone in her position. This is just one example of Joe keeping Janie on a pedestal to show her off to the townspeople. He doesn't allow her to sit outside the store and interact with the common people of Eatonville, for instance, claiming that it is below her. The vultures that descend on the mule's carcass after the funeral symbolize the shallow nature of Joe's actions.
On the whole, Joe is a man driven by ambition. In a white-dominated world, he wants to prove that blacks can be independent and successful on their own. The only way he knows how to do this to become a leader and put the town ahead of everything else in his life. Unfortunately, this includes Janie.