If Applejack was a regular horse and Rainbow Dash was a dog.
Applejack: (Rolling around in the dirt). Oh yeah. The dirt on my fur feels so good.
Rainbow Dash: I don’t understand the point for you horses to just roll around in the dirt.
Applejack: Well why do you dogs chase the mailman?
Applejack is trying to get the horse flies to leave her alone. That’s what horses do in real life. They roll around to get covered in dirt to repel horse flies.
Rainbow Dash: I don’t understand the point for you horses to just roll around in the dirt.
Applejack: Well why do you dogs chase the mailman?