When you set a goal to lose weight, you are focusing on one very small part of the equation, and you are also focusing on a goal that can be pursued to the exclusion of other goals, which might in fact be more important for your health and well-being.
If you are focusing on the scale, you might just fail to notice the inches you've lost, the increased cardiovascular stamina and the other benefits of working out that can sometimes kick in before the numbers really start to drop. Beyond that, weight loss is a goal that doesn't get you very far- even once your weight is down, you'll still need to stick to your workouts and healthy lifestyle in order to maintain what you've achieved, as well as continue to increase your physical fitness.
Consider having a process-oriented goal instead of a results oriented goal. That means that instead of aiming to lose 20 pounds or be able to fit into a size 10, that you instead goal to stick to your schedule of workouts each week and eat a healthy diet at least five days per week. The result is that you are focusing on creating healthy habits that will benefit you for years to come and your success will be doubly rewarding when you achieve your goal AND discover that you've lost weight.