Stress
It hits you from a few angles. First, there's the stress of just…well…dying.
You've seen the labeled driver's ed cars ferrying teenagers around your neighborhood. More or less, the worst thing that happens is that the instructor just says "pull over." And the poor instructor looks seventy-three years old even though he's only forty.
Now take the same thing and put it up in the air at 5,000 feet, traveling 150 knots, and the carrying the souls of a hundred people who've been killed because your student lost focus. It's stress that haunts you, even when times are good.
Then there's the stress of the money. First, think about the civilian who hopes to learn from you. He's already paying $150 an hour for plane rental, not to mention the cost of gas and instruction. It adds up to being a very expensive endeavor. And there's always a younger, cheaper flight instructor willing to do it for less.