I’m having a bit of trouble comprehending the fact that, in the Puritans’ belief, as soon as they entered the world, it was already decided whether they would go to Heaven or Hell – but they still spent their lives being "good" and trying to please God. Why would they do this? I was trying to establish a connection between the Puritans settling in America and what is today known as the American Dream – but this question is kind of in my way.
When you ask this question, you need to know about the elect, those predestined to enter heaven in Calvinist tradition. While John Calvin and others repeatedly said only God knows who is among the elect, he did define the characteristics of the elect. Those included piety, humility, hard work and other traditionally Puritan values.
If you followed the values of the elect, it therefore followed that you must be among the elect. If you rejected those values by dancing or going to theaters, you were clearly exhibiting characteristics that said you were not among the elect.
It's kind of a backward-building construct that way.