What are the two main reasons people become overweight?

This may help:

When glucose reaches the small intestine (after being eaten and then digested), it is absorbed through the lining of the small intestine and ends up in the bloodstream.

When the blood contains the right amount of glucose, the hormone called insulin (from the pancreas) stimulates the liver to convert the excess glucose (which is not needed for the moment) to glycogen and store it in the liver.

If then, more glucose is needed in the blood, another hormone stimulates the liver to release the glucose that it had been storing back into the blood.

This is how the body regulates blood sugar concentration.

However, if there is no more space in the liver for more glucose to be stored, any extra glucose is converted to fat and stored in special fat storage cells throughout the body.

Of course there are other reasons for picking up weight such as lack of exercise, but eating unhealthy food (too much glucose/sugar!) can not only cause people to gain weight, but also cause health problems too. This is why it's so important to eat healthily and to keep exercising.