The degrading probability that you'll return a workout regimen after taking a day off.
The day after break from working out, the memory of going to the gym and not going to the gym are both fresh in your mind. Not going to the gym is a slightly fresher memory so I’ll say that getting a workout in has about a 40/60 chance of happening: second day syndrome. On the next day the probability drops to 30/70. Add that to the guilt of not having gone now for 3 days and you have a recipe for disaster.