Nobody wants a double chin, but a lot of us have them. Even people who aren't overweight! If you have a double chin, I can imagine you probably don't want to have it anymore.
The biggest reason people get them in the first place is due to weight gain. Sometimes it's just because we get older and our skin gets a little more loose.
Gosh darn you, gravity! Muscles that aren't used often enough lose their tone too, and that includes the muscles in your neck and jaw. But it doesn't matter how, it's just that you have it.
Try this simple exercise to help get rid of it. You can work out each muscle in your face and it'll help you get rid of it: