Wanting to fly when he was young, Milkman actually experienced a feeling of passion before in his life. Sadly, this would be the only time he would experience this feeling again. After realizing his impractical dream of flight, Milkman loses a sense of direction. Without any objective, he doesn't know what to do with his life. Even worse, he is reluctant to change; he is still stubborn when even his closest people ask him to do something. Was the realization of the impossibility of flight too much for Milkman? It's so peculiar that at such a young age he had this drastic alteration. What ultimately gets him going is his search for his heritage and his family's origin.
Macon Dead III is probably one of the most peculiar characters that I've ever seen. But we've seen this type of character before. Monsieur Meursault? Lacks passion and emotion for everything? While people might argue that Meursault's reason for apathy was unknown and unclear, at the end, he does admit that his relationship with God (involving the crucifix) wasn't very good. Unfortunately, Meursault never is able to ever attempt to step out of his comfort zone.
It will forever be a mystery on why Milkman so drastically became uninspired. As readers, like we did for Meursault, we hope for these characters to find a solution in the end.