According to tenets of Islam, disappointment is a sin. Prophet Mohammed (PBUH) prevented people from getting disappointed.
In Quran, it is always instructed to refrain from disappointments as it is the best tool of “Shaitan”. It is also said in “Hadith” to stay away from disappointments because it stops person’s development.
“Shaitan” gets satisfied and become happy when he sees someone disappointed. Whenever someone feels sad, he must recall that Allah (SWT) is there for him and must also recall what he got from Allah (SWT) without even asking. Thus, if one fails to get something then he must not regret and should be thankful to Allah (SWT) as he knows better than anyone what is right and wrong.