Why do we gain weight?
The 'calorie' is a unit of measurement of the energy contained within foods. Living bodies require a certain number of calories each day as fuel. Food calories are metabolized (burned up) by the body to create energy necessary to keep it going. Calories in excess of what the body needs as fuel get stored in the form of fat reserves which buffer the body against the possibility that calories might be hard to get a hold of in the future. Stored fat calories get burned and used up when no food is available and the body must look for alternative sources of fuel.
People gain weight and get fat when they consistently eat more calories than their bodies require to meet daily demands. That excess calories get stored as fat is an adaptive evolutionary response inherited from times not that long ago when food was less abundantly available and people had to work far harder to get a hold of what food was available. People who were able to store food in the form of fat when food was readily available were more likely to survive and reproduce through times when food was difficult to get than were their skinny peers. Because of this evolutionary advantage, our bodies have developed so that it is rewarding and natural for us to eat a lot of food when it is available. Despite being efficient storers of body fat, ancestral humans were not often obese as they had to work hard to eat and in the process burned up what calories they ate. However, the dramatic agricultural and technological changes of the past two thousand years have made food extremely easy to obtain and evolution has not been able to keep pace in so short a time span. At this time we are required to use our intellect to understand our bodies' instincts and to develop a more twenty-first century appropriate relationship with food.