It hits you from a few angles. First, there's the stress of just like…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 73 years old even though he's only 40. Now take the same thing and put it up in the air at 5,000 feet, traveling 150 knots and the crying souls of a hundred people who have 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 then gas then YOUR time. It adds up to being a very expensive endeavor. And there's always a younger, cheaper flight instructor willing to do it for less.