On Friday I had the unfortunate circumstance where I had to cancel game. 2/6 people couldn't make the game, and while the rule is normally 50% for this session I didn't feel good going in with 2/3 of the playergroup. The key problem being that the two missing PCs were the ones with the significant plot beats coming up, and combat having the potential to be lethal without the PCs full toolkit available.
Objectively it was the right choice to make. Subjectively I feel like crap whenever I cancel game.
It does make me curious though: under what criteria do you cancel game? How does it make you feel? Do you consider that feeling when considering if the game is fun/good for you? How does it work?