A lot is known about what causes obesity. The simplest explanation is that the genes that have protected us from famines for millions of years are at the core of the cause of obesity. These powerful biochemical systems are centered on a small area in the middle of the brain called the hypothalamus. A specialized area in the hypothalamus, called the arcuate nucleus, is where the signals that control metabolic rate, hunger and satiety are located.
In the arcuate nucleus are two cells types. One cell is the NPY/AGRP (Neuropeptide Y/Agouti Related Protein) cell. This is the hunger cell. If it is stimulated you feel hungry and your metabolic rate drops. The other cell is the POMC (Proopiomelanocortin) cell. This is the satiety cell. If it is stimulated you feel full and your metabolic rate increases.
The two major hormones that control the hunger and satiety cells are ghrelin and leptin. Ghrelin is produced by the stomach and travels to the brain where it turns on the hunger cell and turns off the satiety cell. Leptin is produced by the fat cells of the body and travels to the brain where it turns on the satiety cell and turns off the hunger cell. When you are obese you have lots of fat cells and thus your leptin levels are high. The high leptin levels in combination with high insulin levels turn off the stomach cells that make ghrelin so your ghrelin levels are low. Now if that seems backward, your right. The problem is that high leptin levels over time make the satiety cell insensitive to leptin and low ghrelin levels make the hunger cell hypersensitive to ghrelin. The result is that even though the leptin levels are high and the ghrelin levels are low, the hunger cells are turned on and the satiety cells are turned off.
This is exactly the situation that occurs in a famine, hunger cells on and satiety cells off. So if you are obese, even though you have plenty of fat stores, the brain behaves like you were starving. This has many consequences. First and foremost is a loss of the conscious signals that tell you when you are full and when you are hungry. A common sign of this is that most obese people don't feel hungry when they get up in the morning. As a result they often skip breakfast. The brain interprets this as more starvation signals and further shuts down the metabolism. In fact the number one risk factor for obesity was skipping breakfast.
Ghrelin is also important in many other functions of the body. One of the most important is sleep. In order to efficiently progress though the normal cycles of sleep you need adequate ghrelin levels. If you don't have them you will not sleep as efficiently, you will dream less and get less restorative sleep. This will make you more tired the next day and since dreaming promotes leptin production, you will be hungrier and have a lower metabolic rate.
The imbalance of leptin and ghrelin are at the heart of the cause and consequences of obesity. To see all that this imbalance effects see the sections below.