What is the one permanent ambition of the narrator and his boyhood friends?

In Life on the Mississippi, Mark Twain states that his overriding ambition as a boy was to be a steamboatman, an ambition he shared with his friends. To be sure, these young lads weren't exactly career-orientated at such a young age, so they thought about alternative occupations such as circus clown or pirate. (The latter, of course, is reflected in the exploits of Tom Sawyer and Huckleberry Finn, who love nothing more than playing at pirates). But although young Sam Clemens and his friends may dream about what other kinds of career they might pursue when they get older, their thoughts always return to being steamboatmen.
This isn't really surprising when you consider just how important a role the steamboat played in the town of Hannibal and countless other communities along the Mississippi. Steamboats brought with them mail, goods, news of the outside world. They pointed to the existence of a much bigger world outside, a world that seemed so exciting to youngsters like Sam and his friends. Not only that, but steamboat pilots were relatively well-paid; they could earn in two months what a preacher would make in a whole year.