Resigned, Befuddled, Depressed
It's important, we think, to distinguish the fact that there are more like two voices narrating our story: the adult narrator taking a trip down memory lane, and then the view of the boy as the events are occurring.
The adult voice from the prologue and epilogue is clearly a dude who could use a little sunshine in his life.
I would talk to people whose existence I had forgotten years before and they would ask me about my marriage (failed a decade ago, a relationship that had slowly frayed until eventually, as they always seem to, it broke) and whether I was seeing anyone (I wasn't; I was not even sure that I could, not yet) and they would ask about my children (all grown up, they have their own lives, they wish they could be here today), work (doing fine, thank you, I would say, never knowing how to talk about what I do. If I could talk about it, I would not have to do it. I make art, sometimes I make true art, and sometimes it fills the empty places in my life. Some of them. (Not all.) (Prologue.8)
Wow, who is he, Eeyore? In fairness, he's not always gloom and doom, but it's definitely clear that he's a guy who has a hard time finding contentment with his life, and he's resigned to this miserable fate.
This tone isn't totally absent from the voice of the boy, although it must've taken a few years to hone into the Debbie Downer perfection it's attained in adulthood. He says:
I was not happy as a child, although from time to time I was content. (2.1)
Gee, that's uplifting. So during his friendship with Lettie and their adventures together, his tone is best described as depressed but befuddled, like, "I'm not sure why I'm bummed out, I'm just kinda confused about it."
The reason this sad emptiness is important to the story is that it makes us wonder: Do the hunger birds truly eat his heart? Does Ursula leave a hole where his heart is supposed to be, and that's why he's always so miserable? Or is it just life getting him down? And, as is often to case with this book, we're really not sure.