Usually, where there is a problem with weight gain, which leads to health issues, there's a psychological cause. I understand that this isn't the case for everyone, though. Sometimes it can be a control issue i.e. a person cannot control their environment, but they can control what they eat. Other times it can be a comfort issue i.e. a person was fed as a child by someone they loved and associates food with the comfort of love.
I'm not perfect by the way. I've got a very strong addictive personality and I've been battling it for years. Since I learned that I'm always scared that I'll miss out on something (because of my childhood) I've been getting better at battling it. Us humans can be so complex!