People, on average, eat until they're no longer hungry. Problem is, there's only a loose relationship between your caloric needs and your hunger response. That's how you end up with underweight people who are trying to put on muscle saying they can't possibly eat any more and still can't put on weight, while having overweight people who eat twice as much as that guy and have to actively choose not eat more. Both people can make a conscious choice to disobey their signals, just like how you can choose to hold your hand to a hot stove. But it takes a lot of energy to keep up that willpower. Effective weightloss drugs solve that problem, by treating the actual problem: the hunger.