Disclaimer: I'm not an expert on this, just describing my own experience (which may be subjective/biased/completely misinterpreted by me).
If you're mostly sedentary in a first-world country, going off sugar/carbs-dominated diet brings your insulin sensitivity to its natural levels (which, very probably, it hasn't been since the moment you started eating adult food). For me this had the amazing effect of becoming more alert and productive mentally and more aware of my body physically. As an athlete, I suddenly discovered that I can be underperforming because I'm lacking fuel, and not because I'm tired, and that if I "fuel up" by eating carbs, I can expect a surge of energy that will allow me to perform well. Needless to say, the discovery was amazing, I felt like a finely tuned sports car or something.
Back when I was eating a regular western diet, my insulin sensitivity was low, and "injecting" fast carbs made me drowsy, not full of energy.
Needless to say once I experienced that new state, I got hooked, so sticking to good eating habits haven't required tremendous will power for me. Getting there, though, did.