The water from the Tucks' spring gives the gift of immortality, of everlasting life. To most people, this sounds amazing. Just imagine all the wonderful things you could do, all the places you could go, safe in the knowledge that, whatever happens, you're never going to die. But in reality, immortality is a mixed blessing to the Tucks, as Winnie discovers.
Jesse, for example, had never experienced much of life before drinking from the spring, and so he'll remain as a seventeen year old for the rest of eternity. This means that he'll never get to grow as a person, either physically or emotionally. And Angus, Jesse's father, is pretty bored of the whole immortality thing. He illustrates the old adage that all joys are mortal; that we can only truly enjoy life in the knowledge that one day it will all end. For Angus, however, each day is just the same old same old, and with literally no end in sight, we can see why immortality is as much a curse as a gift.