Can't You See? He Wants to Live.
The title of the book is also its very last line. In case you forgot, it's "I know this much is true" (48.47). But what? What does Dominick know is true? That what goes up must come down? That e=mc2? That William H. Macy should have beat Cuba Gooding Jr. at the 1997 Academy Awards? Here's the novel's penultimate (i.e. second-to-last) paragraph for your consideration:
I am not a smart man, particularly, but one day, at long last, I stumbled from the dark woods of my own, and my family's, and my country's past, holding in my hands these truths: that love grows from the rich loam of forgiveness; that mongrels make good dogs; that the evidence of God exists in the roundness of things. (47.47)
So what the heck is he talking about here? Well, as much as Dominick wants to take care of his brother, his main journey is getting over his own anger, and his anger results from his martyr complex. Why has God chosen him to be his brother's caretaker? What about Dominick? It isn't fair. He's had enough and now he wants his share.
It takes the death of his brother for him to realize that the universe isn't actually out to get him. He can be his own person, and not a victim of his own history and circumstance. He can learn from the past and the mistakes he and his ancestors have made—you know, falling off a building, letting pride and arrogance get in the way. Also, he's learned that he can be as big of a jerk as he wants to be, but as long as people forgive him, it doesn't matter. He treats his wife like poop, and she remarries him. That's love, man. (Or is it?)
Dominick also says, "This much, at least, I've figured out" (48.47), which implies that he also knows that he can never know everything. There is always more to learn, but at least he's finally made some sense of his life so far.