The answer can be found in Chapter 8 of the novel.
Mattie Lou feels that, because the body is home to the mind and soul, that it is sacred and in death deserves respect.
"A nice funeral is sort of a thank-you," she added. "A person's body oughtn't be treated like no old dead dog."
Rucker Blakeslee disagrees, arguing that the body should be put directly into the ground "as the Lord intended" and telling his wife that he'd like a party when he died rather than a funeral. (He gets his wish at the book's conclusion.) To Will, he later adds that funerals are for hypocrites who are "mostly bein' glad to be alive theirselves and tryin' to pretend they ain't havin' a good time seein' one another."
The fact that he honored Granny's wish for a nice funeral rather than characteristically doing as he himself wanted proved to Will that Grandpa did indeed love his wife deeply.