What is the best theme for this novel?

One of the primary messages in Tuck Everlasting is that every living thing must die someday, and that death is actually not such a bad thing, because you have to die if you are going to live. Learning this lesson helps Winnie take the first steps on the road to adulthood, allowing her to speak truthfully and to exercise compassion to other living creatures.

The Tuck family is not actually thrilled by their immortality. For example, Miles loses his wife and children when they grow suspicious of his immunity to aging, and Jesse longs for a wife to enjoy the world with. However, Angus Tuck remains the most vivid example of the discontent with immortality, especially when he gazes almost longingly at the near-dead form of the man in the yellow suit, implying that he actually actively wishes for death (p. 101).