This story is a very sensitive and touching tale of one boy's growth from a child to an adult. The way that the story follows a year in the life of this boy and in particular the relationship that he develops with his pet capture what it is to grow up in such a difficult setting. If it is a moral that you are looking for, I think the moral is best expressed in the following quote, which captures the essence of what Jody discovers through the pages of this story:
You've seed how things goes in the world o' men. You've knowed men to be low-down and mean. You've seed ol' Death at his tricks... Ever' man wants life to be a fine thing, and a easy. 'Tis fine, boy, powerful fine, but 'tain't easy. Life knocks a man down and he gits up and it knocks him down agin. I've been uneasy all my life...I've wanted life to be easy for you. Easier'n 'twas for me. A man's heart aches, seein' his young uns face the world. Knowin' they got to get their guts tore out, the way his was tore. I wanted to spare you, long as I could. I wanted you to frolic with your yearlin'. I knowed the lonesomeness he eased for you. But ever' man's lonesome. What's he to do then? What's he to do when he gits knocked down? Why, take it for his share and go on.
Jody moves from being an easygoing child who has a very close relationship with nature and in particular with his pet to recognising that he has entered an adult world of harsh choices and difficult decisions. This is of course shown most powerfully when Jody has to decide to kill that which is most important to him. The novel above all shows that adult life is a battle for survival, and that this comes before any other emotion such as love.