First off, if you feel frequent disappointment then you are in danger of health risk. Studies show that this emotion is directly tied to stress and a bunch of related behaviors that are bad for your health.
This is the important part. What causes this emotion? Simply defined, disappointment occurs when your expectations are not in line with reality. Usually, and get this, it is the fact that your expectations are not realistic and NOT because the universe has not aligned to meet your expectations.
This is the most important question to ask when you feel this emotion of disappointment. Are my expectations realistic?