On a personal level of willpower and motivation, yeah, losing weight can be pretty difficult, But behavioral change has been nailed down to a science. Ever hear of operant conditioning?

Operant conditioning states that behaviors are increased by a reward of positive stimuli or removal of negative stimuli (reinforcement) and decreased with an additional of negative stimuli or removal of positive stimuli (punishment.)

Naturally, this effect is only achieved if the rewards/punishments are of value to the person.

In this sense, if someone reduces their calorie intake (a target behavior) and they lose weight, they'll be far more likely to maintain their dietary changes if that weight loss (or any added positive stimuli like clothes fitting better, being perceived as more confident/attractive and removed negative stimuli like joint pain, sleep problems, exhaustion) is valuable to them.

I will say however that the behavioral changes associated with weight loss are compromised by the fact that the reinforcing stimuli (weight loss, whathaveyou) are on a delayed timeline. The longer you wait to reinforce/punish a desired behavior, the less effective the stimuli will be in securing long term changes.

This is particularly true when countered with an immediately reinforcing stimuli like, oh say, food. Why engage in a behavior that is only rewarded after an extended period of time when you can use an opposing behavior that will reward you right now? If someone values bingeing on cupcakes and getting a dopamine rush more than they value health/weight loss, the odds are not in their favor. It's no excuse, but it would take some work to overcome.

Okay, I'm done psychobabbling now.