The Tucks think of immortality as a gift because it protects them from death and aging. They have lived long enough to witness numerous historical events and technological advances. However, eventually, they discover that eternal life robs them of human experiences, such as growing up and having children. They yearn for mortality, which they begin to view as a privilege rather than a curse.
The irony is that the Tucks have found themselves in a paradox: they desire to be mortal despite having gained immortality. The book explores themes such as the value of life, the passage of time, and the human experience by emphasizing this irony. Ultimately, it emphasizes the bittersweetness of life and suggests that despite its difficulties, growing older and dying are valuable aspects of human existence.