"It's just really odd. My whole life I've thought, I'm gonna work on Wall Street. My mother did it, and she loved it, and then kind of realizing, Maybe I'm not my mother," he said.
He admitted that, after six years, he remembered less of his mother. But he was also letting go of his anger.
"I used to be angry at Osama bin Laden, but now that just seems so absurd to me," he said. "I don't really know him -- I guess I don't really hate him."
By 2009, Chirls had begun coming to terms with his father.
"Somehow, we had dinner together. And it eventually became a weekly dinner over the past year. I think I held him to the unrealistic standard. You know, he's my father, but he's also just a human being, a guy that lost his wife and his, his best friend," he said.
Today Chirls is a writer -- he's working on a book about the aftermath of his mother's death -- and an entrepreneur. He founded a company that helps would-be volunteers with specific goals find those in need of their services.
As he chronicled Chirls' life, Whitaker said it was hard not to step in and offer the young man some guidance, but "it wasn't the right thing for me to do. It wasn't my place."
Instead, Whitaker said, he watched Chirls make his own decisions and move through his life with grace. He hopes viewers are inspired by Chirls and the stories of the other four people featured in "Rebirth."
"By seeing the film," he said, "you may recognize that this [healing] process is a long one and how resilient human beings are."
"Rebirth" is currently in theaters and will appear on Showtime Sunday night, the 10th anniversary of 9/11. For more information, visit projectrebirth.org