All of the themes in Steinbeck's "The Red Pony" revolve around painful lessons of maturity.
In the first section, "The Gift," Jody learns that gifts come with strings. "Nearly all of his father's present were given with reservations which hampered their value somewhat. It was good discipline." Jody learns that even the gift of friendship with ranch hand Billy Buck does not come without reservations. Jody trusts Billy completely but utter faith in humanity is destined for disappointment.
In "The Great Mountains" Jody begins to question what lies beyond his immediate home. "Do you know what in the big mountain?" he asks his mother and Billy Buck. Their cyncial replies do nothing to dampen his curiosity; instead, it feeds it. Growth requires questioning of authority.
The final section, "The Leader of the People" brings Jody into his own as a man. Here, he learns empathy and respect. When his grandfather is disrespected by his father, Jody tries to soothe the old man's feelings. The final lines prove this at least to his mother. Jody refuses lemonade for himself and wants only to make his grandfather feel better. He has grown as a human being.