People gain weight when the body takes in more calories than it burns off. Those extra calories are stored as fat. The amount of weight gain that leads to obesity doesn't happen in a few weeks or months. Because being obese is more than just being a few pounds overweight, people who are obese have usually been getting more calories than they need for years.
Genes - small parts of the DNA that people inherit from their parents and that determine traits like hair or eye color - can play an important role in this weight gain. Some of your genes tell your body how to metabolize food and how to use extra calories or stored fat. Some people burn calories faster or slower than others do because of their genes.
Obesity can run in families, but just how much is due to genes is hard to determine. Many families eat the same foods, have the same habits (like snacking in front of the TV), and tend to think alike when it comes to weight issues (like urging children to eat a lot at dinner so they can grow "big and strong"). All of these situations can contribute to weight gain, so it can be difficult to figure out if a person is born with a tendency to be obese or overweight or learns eating and exercise habits that lead to weight gain. In most cases, weight problems arise from a combination of habits and genetic factors. Certain illnesses, like thyroid gland problems or unusual genetic disorders, are uncommon causes for people gaining weight.
Sometimes emotions can fuel obesity as well. People tend to eat more when they are upset, anxious, sad, stressed out, or even bored. Then after they eat too much, they may feel bad about it and eat more to deal with those bad feelings, creating a tough cycle to break.
One of the most important factors in weight gain is a sedentary lifestyle. People are much less active today than they used to be, with televisions, computers, and video games filling their spare time. Cars dominate our lives, and fewer people walk or ride bikes to get somewhere. As lives become busier, there is less time to cook healthy meals, so more and more people eat at restaurants, grab takeout food, or buy quick foods at the grocery store or food market to heat up at home. All of these can contain lots more fat and calories than meals prepared from fresh foods at home.