I spoke to a group of entrepreneurs at the University of Illinois yesterday. It was really cool for me to be among a crowd of people who launch and lead startups.

As I thought about what to highlight from ten years as a serial entrepreneur, I noticed that the hardest parts and the biggest lessons I learned were all related to human factors. These were things like wise hiring choices, tough firing conversations, trust and ethics in partnerships, the emotional drivers of customers, and what makes a high-performing team.

Being smart and working hard are valuable ingredients in entrepreneurial success, and they are not enough. The human factors take things like emotional insight, courage, honesty, kindness, and generosity to navigate successfully.

I’ve learned recently in my pilot training that 85% of aviation fatalities are human error in a perfectly good airplane. I think it’s the same in business as in aviation, the human factors cause most of the failures.

The difference is control. As engineers we control software and materials and processes. As leaders it’s totally different. We work toward goals we can’t achieve on our own, with people whose help we need and whose behavior we do not control.

As leaders we must learn how to seek mutual benefit and interact with appropriate emotional adeptness or we won’t be going far on anything other than a solo project.