It's a fall evening in Tokyo, and Nikesh Arora is supposed to be in two places at once.

This is a common dilemma for the 47-year-old president of SoftBank Group Corp., who splits his time between Silicon Valley, Japan and India. Arora is scheduled for dinner with his boss, Masayoshi Son, but he keeps talking. He has a story to tell.

Arora is explaining how he came to make one of the largest executive stock purchases ever. It began late one night this year when he and Son were talking about people's tolerance for risk and how it tends to decline over time. Arora took a chance as a kid by leaving India for the U.S. with only $200 in cash, but he had since gone on to a lucrative career. So Son prodded him.