One might expect a ball to bounce towards you, but instead it goes in a different direction. Something a person expects to be of some benefit could turn out to be of little value. Or, maybe someone a person likes prefers someone else. So, if you experience something like that, a person could speak the sentence in red to you.
事與願違
當你期望事情的發展與現實結果相反、不符合時就可以這麼說。有些人期望可以得到更大利益或者心想事成但總是沒有想像中順利。