What evidence is there that Grandfather enjoyed his early years as a soldier?

Mattie's grandfather, William Farnsworth Cook, served under George Washington in the Pennsylvania Fifth Regiment. He's one of Mattie's mentors and readily passes on some of the skills he learned as a soldier, which prove vital to Mattie at various points throughout the story. Many old soldiers refuse to talk about the conflicts in which they've served, on account of the horrific sights they've witnessed and the deep trauma they've suffered. But Mattie's grandfather appears to have enjoyed his years as a soldier. He shows this, for example, by dressing up in his old army uniform when he escorts Mattie to the countryside. As mentioned earlier, he also teaches Mattie some of his old soldiering skills. Serving as a soldier under Washington gave Grandfather William a real sense of direction in life. To some extent, his work at the Bush Hill Hospital gives his life as a civilian a similar overriding purpose.