The 47-year-old actor - who raises six children with partner Angelina Jolie - feels he used to be more "optimistic" when he was younger and has grown more jaded with the world with age.
He said: "I was more optimistic and naive when I was younger and I arrived in Los Angeles with a few hundred dollars in my pocket and took any odd job I could find, then worked as an extra before I started to get acting jobs.
"I gradually grew more cynical and realistic about the world and in some ways I`m sorry I lost some of that openness and enthusiasm."
However, Brad admits being a father to kids Maddox, 10, Pax, right, Zahara, six, Shiloh, five and three-year-old twins Knox and Vivienne has helped him regain some of his youthful zest for life.
He added: "Being a father and being part of my own family has rekindled a lot of that lost spirit, although I still have a naggingly realistic outlook on things."