1. Genetics
Obesity has a strong genetic component. Offsprings of obese parents are much more likely to become obese than offsprings of lean parents.
This is not to say that obesity is completely predetermined because our genes aren’t as set in stone as you may think… the signals we send our genes can have a major effect on which genes are expressed and which are not.
Non-industrialized societies rapidly become obese when they start eating a typical Western diet. Their genes didn’t change, the environment and the signals they sent to their genes changed.
It does seem clear that there are genetic components that do affect our susceptibility to gain weight. Studies on identical twins demonstrate this very well