As long as the marginal benefits are higher than the marginal costs you are better off continuing the activity.
Consider the example of eating pizza. Each slice of pizza gives you happiness and helps fill you up (marginal benefit), but each slice also has lots of calories and fat (marginal cost). As long as you are still hungry and getting enjoyment from eating, you should keep eating. But once you reach the point where you are too full then you should stop, because the costs now outweigh the benefits.