Angus feels as if the Tucks are not fully participating in life because they are immortal.
Angus uses the rowboat outing to explain to Winnie how he feels about being immortal.
That's what us Tucks are, Winnie. Stuck so's we can't move on. We ain't part of the wheel no more. Dropped off, Winnie. Left behind. And everywhere around us, things is moving and growing and changing. (Ch. 12)
Angus considers the Tuck family “stuck” in time. Because they cannot grow old or die, they are isolated from other people. They can’t really make friends. They definitely can’t have families—at least Jesse and Miles can’t. People become suspicious of them and think they’re freaks.
Tuck uses the metaphor of a wheel to explain the concept of living forever to Winnie.
But dying's part of the wheel, right there next to being born. You can't pick out the pieces you like and leave the rest. Being part of the whole thing, that's the blessing. But it's passing us by, us Tucks. Living's heavy work, but off to one side, the way we are, it's useless, too. (Ch. 12)
Although the idea of living forever sounds great, Tuck feels that life is passing them by. He explains to Winnie that while she may not want to die, dying is a part of life. If you can’t die, are you really living?
The Tucks give Winnie the option of drinking from the spring, but they want to make sure she understands what she is getting into. If she drinks from the spring, it will be because she had information the Tucks did not when they drank. She would be choosing to remain a little girl forever.
Jesse wants Winnie to drink when she is older, so that they will be the same age. He wants a companion. Although Winnie cares for Jesse, we know that she does not choose this choice. The Tucks return to find her gravestone. Faced with the choice of living forever in an isolated existence, Winnie chose mortality.