Different people have different reactions to food. Some people simply have food addiction - which could be a product of environment (eg eating as a reaction to stress) or they may be victims of faulty genes which cause their neural "award systems" to over activate when they eat. I've only studied this a little bit to be honest so I can't give you exact example of which genes. I'm not that well read on brain neurology. Some people also have faulty versions of genes whose products function to tell you when you're full (eg, ob gene mutations -
http://en.wikipedia.org/wiki/Leptin , leptin receptor mutations, mutations to the ghrelin gene).
Essentially, if the genes of biochemical messengers which control satiety are malfunctioning (or their receptors) you will see people over-eat or potentially under-eat aswell I suppose. Other examples are: orexin, and PYY 3-36, cholecystokinin, any of the hunger/satiety hormones.