Why had Tuck been smiling?
The answer to your question is found in chapter two of Natalie Babbitt's Tuck Everlasting.
Angus Tuck is smiling because he is having a good dream.
He snored gently, and for a moment the corners of his mouth turned upward in a smile. Tuck almost never smiled except in sleep.
A few moments later, Angus Tuck tells his wife, Mae, that he was having the good dream about being in Heaven.
"I was having that dream again, the good one where we're all in heaven and never heard of Treegap."
The reason that Tuck thinks that being dead and in heaven would be so great is because it is something that will never happen to him. Tuck is an immortal, and he became that way by drinking from a magical spring in the Treegap forest. More than anything else, Angus Tuck wishes that he could die. By being able to die, he would be truly living.
"If I knowed how to climb back on the wheel, I'd do it in a minute. You can't have living without dying. So you can't call it living, what we got."
The dream is a good dream and causes him to smile, because by being dead in the dream, it means that he truly lived.