I left my 10-year career at NASA because I wanted to commercialize some of my patents, and in order to do that I had to learn how to run a business.
The company I started was called E2 Solutions (E2 stood for "Extended Enterprise" - we were an Information Technology consulting firm which had visions of using the new internet to enable remote work from anywhere.) Our claim to fame was being a "Systems Integrator" - we could hook up your PC, Mac, and Unix boxes on the same network and have them all talk to each other. We sold computers and services to customers like the Army, Air Force, NASA, Dole (the pineapple company), just to name a few.
The company started with me and a telephone, and I grew it to 22 employees and $10M in revenues in just 3 years. In five years it crashed and burned because our employee expenses kept going up but our revenues stayed flat - I blame the ineffectiveness of our sales staff.
I learned a lot, but it was a very stressful 5 years.
After that I went to work for an internet-based insurance company during the dot-com boom. Made a lot of money then lost a lot of money during the tech crash of 2000. Tried to raise money to start a new company to commercialize one of my patents (using the Data Egg typing scheme and grafting it onto cell phones. Texting was a new thing then and all people had to type with were numeric keypads. You had to press a button 3 times just to get a "C"!!) After 18 months I plowed through my life's savings. Giving up, I said, "Screw it!" and went to China to teach English for a semester.