Symbolism, Imagery, Allegory
At first, Dave thinks that his only problem is that people treat him like a kid. As his experience with Jenny the mule shows us, however, Dave's problems might run even deeper than that—people don't just treat him like a kid, they treat him like an animal.
In fact, Dave alludes to this himself when he says that "they treat [him] like a mule, n then they beat [him]" (206). While it's true that Dave acts immaturely throughout the story, he deserves a whole lot more respect than he is given. Dave can empathize with poor Jenny because they both work all of the time and get no respect. And after accidentally killing Jenny, Dave is poised to work for no pay, too, just like the mule.
Of course, Jenny's death complicates things. For the first time, Dave realizes that his life might not end well if he stays where he is. True, he probably won't get accidentally shot while working in the fields, but this is a metaphor, people. If nothing else, the incident with Jenny makes it abundantly clear that Dave's life is a dead-end as long as he remains in his hometown. Fingers crossed he finds something better elsewhere.