In Chapter Four, Mark Twain tells us he always wanted to be a steamboat pilot; it was his childhood ambition for as long as he could remember.
He tells us that, as time progressed and all his boyhood friends grew up, many heeded the call of the river by working as ship engineers, mud-clerks (general workers who performed the dirtiest jobs on a steamboat), and pilots on the Mississippi River. Mark Twain asserts that a steamboat pilot's pay was in itself a great lure; every pilot was paid between $150 and $250 a month, a "princely salary" at the time. Additionally, the steamboat pilot never had to pay for board, and his salary easily eclipsed that of a preacher's. To become a steamboat pilot, one had to train as a cub-pilot. This is partly why Mark Twain was driven to become a cub-pilot; without the training, he would never have realized his ambition of becoming a steamboat pilot.
Mark Twain also liked the idea of traveling on the water; such an adventure exhilarated him and gave him a profound sense of well-being and vigor. On a ship bound for New Orleans, he wrote,
When we presently got under way and went poking down the broad Ohio, I became a new being, and the subject of my own admiration. I was a traveler! A word never had tasted so good in my mouth before. I had an exultant sense of being bound for mysterious lands and distant climes which I never have felt in so uplifting a degree since. I was in such a glorified condition that all ignoble feelings departed out of me, and I was able to look down and pity the untraveled with a compassion that had hardly a trace of contempt in it.
So, basically, Mark Twain was driven to become a cub-pilot because of a boyhood ambition to work as a steamboat pilot and a desire to reap the material benefits from such an occupation. Additionally, he also viewed a life on the water as one replete with adventure and excitement.