Well it really depends how you look at it. On the one hand, it is an adult's choice to eat or not eat. On the other hand, that same adult was once a kid who was exposed, from birth, to corn syrup infused baby formulas, and convenience food, and particular patterns of activity and eating. Psychologists find that it is extremely difficult and, in many cases, impossible to undue the damage of experiences and conditioning that we were exposed to from infancy. These eating and activity behaviors, for many of us, were developed as our personality developed.
So, superficially, it is a choice. But it can be a constant struggle to choose conscious choices over the more deeply ingrained subconscious ones.
As far as genetics go, even if you did naturally have a slower metabolism, you can still lose weight. It is just a matter of recognizing yourself as a unique individual with different calorie requirements than those around you.
There is no better feeling in the world than starting off thinking you can't do something and you do it anyways-Dolvett <3