We just remembered some more discriminators. Your brain needs to be about 20 seconds ahead of your mouth, and about a minute ahead of the participants. Adults are very demanding learners. Sick days apply to other people. There simply isn't any such thing as a 'stand-in'.
Headaches have to be managed, as does the 'flu.
If you don't like flying, forget it. Oh - if you're not travelling, writing proposals or creating some fancy new overheads, you just might have time for your family. That's about it. There are trainers out there to whom all the above doesn't apply. And we're sure they're very happy too.