Fair point.
A person can force themselves... until they break.
So in that sense it needn't be fun as long as you can finish the task before the breaking point.
But there's a threshold beyond which the person cannot sustain the behavior.
Because it's a form of consistent lying to oneself.
A person's brain, on a moment-to-moment basis is trying to pick the 'optimal' choice. The 'optimal' is: most pleasure, least pain.
If the most pleasure, least pain option is beneficial to you in the long-run, the "game" that we play of trying to convince ourselves to do something else disappears.
Which diet will win in the long run:
1. One that tastes better than all available options (including any takeout, or delivery, ice cream, or restaurant you know of)
2. One that tastes worse
My critique is with this flawed concept of "discipline" which to my mind translates to "forcing yourself" aka "lying to yourself for as long as possible"