Long answer: us kids were starting high school and he knew college was after that, so he needed to figure out how to come up with some money to help us. He was good at math, so someone suggested he play the stock market.
He started off investing "fake" money (just his own personal ledger) and did pretty good. So he invested small amounts and continued to do well. He finally decided to borrow money from the bank, which is how the story came about.
He actually did REALLY well; paid off the loan, got a bigger one, paid that off, etc, until he had enough of his own money to keep investing. He beat the S&P every year, he had a talent for it.
But he really didn't enjoy it, he said it was a lot of work to keep up with the markets and how they interacted with each other. He kept it up for a few years after we finished college until he had a comfortable nest egg and quit.
He only paid for half our college, as a matter of principle he wanted us to come up with the other half. I still have loans to pay off but it was a huge help.
Now he hates capitalism and doesn't do any investing at all. He used to have some safe mutual funds but he's jaded about the state of the world, he doesn't want any part of the system.