Because you're feeling peckish. Because you hadn't had a good burger in a while. Because you saw a good looking pizza on TV and you can't order less than a full pizza. Because your mother made this good lasagna and would feel offended if you didn't eat a portion because you'd already eaten earlier. Because you're sadand tired and just don't want to also feel hungry, and eating a pack of oreos dipped in nutella is more pleasant than cooking a steak. Because you're feeling self-destructive and it feels good to ruin your diet and get that feeling of satiety that you've been denying for a while.

There are a million reasons like this. You need quite consistent motivation in order to not overeat; overeating is the default for most overweight and obese people.