Training
Genetics
Diet
Drugs
For me it is number 3 Diet.
Diet is the hardest part for me. After 2-3 months on low-carb diet even the worst junk food appears as if it has landed from heaven and not even exaggerating.
My physique is more of a gainer, can pack muscle mass and fat fairly quickly. So my body doesn't like loosing weight. I have to go ballistic on calorie deficit to get the fat burning started.