What drove Mark Twain to become a cub-pilot on a Mississippi riverboat in Life on the Mississippi?

Mark Twain’s first into the career of steamboat cub-pilot is well-documented. He was always very interested in the profession and was fascinated by traveling the Missouri River. However, when he was 11, his father passed away and he had to leave school to pursue work to help support his family, derailing his opportunity to legitimately get trained to be a steamboat pilot.
When he was 18, after working as a typesetter, writing articles, and teaching himself at night, he convinced a pilot to take him on as a cub-pilot. The cost of his training was deducted from his salary on the boat, but he enjoyed the work. It was during this time that he finds his pen name, as he was born Samuel Langhorne Clemens. His name, Mark Twain, comes from the boating term for “twelve feet”, the necessary water depth for safe passage.