Harper Lee and Truman Capote became friends as next-door neighbors in the late 1920s, when they were about kindergarten age. From the start, they recognized in each other "an apartness," as Capote later expressed it; and both loved reading. When Lee's father gave them an old Underwood typewriter, they began writing original stories together.
My theory is that Kurt had a lot of residual pain from his childhood. And when you pile that on top of his experience in World War II - he was in Dresden when it was bombed and saw a city annihilated. When you combine those two things, my impression of Kurt Vonnegut at 84 was that he was a very pained and haunted man.