Worry
I love the idea that worrying is futile. Since most of what we worry about never happens we're causing ourselves unnecessary pain, and in the couple cases it does happen we double the pain adding the pain of worry to the pain of whatever happened. (I believe this is a stoic thought.)
I spent time with some people who seemed 'worry-free' and they were constantly telling stories of how things didn't go their way. How they got stuck in various countries or their plans constantly were mixed up. I wonder if they worried more about things going wrong if they would be able to prevent some or many of those. But perhaps it wouldn't be worth it.