Why does the body resist weight loss so much even when someone is eating less and moving more?

The body has strong biological mechanisms that evolved to defend against weight loss, historically as protection against famine. As weight drops, hunger hormones tend to increase, energy expenditure at rest can decrease more than expected from weight loss alone, and the drive to eat can intensify. It's not a willpower failure; it's the body responding the way it's biologically wired to respond to a perceived energy deficit, which is part of why sustained weight loss through diet and exercise alone is genuinely difficult for a lot of people. References: https://obesitycanada.ca/healthcare-professionals/adult-clinical-practice-guideline/