While having a fast metabolic rate isn't necessarily good or bad for your health, it's important to make sure you're eating enough calories to maintain and nourish your body, while striving not to eat too many calories, which can cause an energy imbalance. Having a high metabolism has its benefits; people with a high metabolism tend to burn calories faster than people with a low metabolism. On the other hand, having a fast metabolism can make it difficult to gain weight or maintain a healthy diet. How fast your body's engine works on average over time determines how many calories you burn.
If your metabolism is high (or fast), you'll burn more calories at rest and during activity. A high metabolism means that you'll need to eat more calories to maintain your weight. That's one of the reasons why some people can eat more than others without gaining weight. A person with a low (or slow) metabolism will burn fewer calories at rest and during activity and will therefore have to eat less to avoid becoming overweight.
Maintaining a healthy metabolic rate means staying physically active, following proper nutrition, getting enough sleep, and avoiding stressful situations. There's nothing inherently wrong with having a fast metabolism. However, if you are concerned about your health, such as an inability to maintain a healthy weight, see a doctor.