The metabolism of our body helps in turning the calories into energy which is why, a slow metabolism results in the build up of calories that are later in stored as fat cells in certain areas of our bodies. This build-up of fat cell result in weight gain which in turn, if untreated, turns into obesity.
There are certain factors that are actually responsible for the slow down of your metabolism such as sugary food, which when taken in excess, results in an increase in inflammation which in turn locks the metabolism in a cycle that propagates fat.
Also, lack of sleep is also responsible for the slow down of the metabolism and in turn making you gain weight and thereby suffer from obesity.