I’ve found that the hardest part is usually the first few days. After you’re in ketosis and burning fat (assuming you have a fair amount to lose) it becomes much easier. I find listening to fasting pod casts or youtube lectures about fasting to be very encouraging and helps with motivation when riding out a fast.
Assuming you are in good overall health, It could be detox/withdrawal from sugar and caffeine. after that it is likely a mental game. For me, After 4 days its like someone hitting a switch and I go into this almost euphoric sense of well being. no hunger, no discomfort, no cravings.