Disappointment is when you are unhappy because what you expect didn't happen. And there are two major causes of disappointment:
When you have an unreasonable expectation, then you are the cause of whatever resultant disappointment. And this is the type of disappointment I see often. It is also very common among sports fans. Someone extremely angry and sad because a player continents away missed a goal. Someone refusing to eat because his club lost a match. Isn't there enough troubles we face in Nigeria that I will have to add expectations from someone who doesn't know I exist and makes his money anyway to my list of worries. And that is besides the fact that life is full of ups and downs for everyone (and even every football club). Must I add their downs to mine?
Whenever you have an expectation that is not paying enough attention to reality, then disappointment is guaranteed. There are people who act like everyone owes them. When they ask for a chance to swap to your lane and in front of you, if you refuse they get mad. If they ask you for help and you are not able to help, they feel dejected. If anything goes wrong they feel worried. They are far from reality in interpreting the events that happen to them. No one has the right to place expectations without consent on anyone. You shouldn't expect me to do every of your bidding except we had a prior agreement to that. So rampantly placing expectations on everyone is a sure way to be disappointed.
Someone else. When is someone else the cause of our disappointment? It is when the person breaks his word. If your friend promised to help you with a task and eventually didn't, then the disappointment you experience is caused by him and not you. And this is the only type of disappointment I consider healthy. There is already a mutual agreement about the expectations, and not that only one party knew of the expectation or agreed to it.
Now, how do you handle disappointments?
I like the principle in the following quote by Friedrich Nietzsche -- "I'm not upset that you lied to me, I'm upset that from now on I can't believe you." You learn from it. You separate the cause from the effect. You focus more on "moving forward". Just like Friedrich, he was focused on what will happen "moving forward".