What does Angus think the spring came from?
Chapter seven of Tuck Everlasting tells the story of how the Tucks came to discover the spring, and how they learned that they were immortal. It is actually a rather humorous chapter, because each Tuck takes a turn describing an event that should have ended in death. Angus Tuck even went so far as to shoot himself in the chest with a shotgun.
"Tuck said—that's my husband, Angus Tuck—he said he had to be sure, once and for all. He took his shotgun and he pointed it at hisself the best way he could, and before we could stop him, he pulled the trigger . . .The shot knocked him down. Went into his heart. It had to, the way he aimed. And right on through him. It scarcely even left a mark."
That part is a bit morbid. I mean Angus could have been wrong and wound up dead.
At the very end of the chapter, Miles Tuck says that they have no idea how the spring works or even why the spring works the way that it does. Angus Tuck isn't present at this point in the story, but Jesse tells Winnie what Angus believes.
"Pa thinks it's something left over from—well, from some other plan for the way the world should be," said Jesse. "Some plan that didn't work out too good. And so everything was changed. Except that the spring was passed over, somehow or other."
In other words, Angus Tuck believes that the spring must be left over from the original creation, and it some how avoided being changed when the rest of the world changed.