Only One Side is Willing
Many years ago, a farmer happened to see the King's daughter in the capital. He lost his heart to her at once. After returning, he missed the princess day and night. Soon, he was ill. His friends and relatives came to see him. When they knew the reason why the farmer was ill, they thought it was quite difficult to help the farmer, for the King would never consent to his daughter marrying a poor man. But the sick man sighed sadly:" If I can't get the princess, I will be dead." His friends tried to persuade him, but in vain. So they had to play a trick. Some days later, they came to see the man again and told him they had asked the princess to marry him, but she rejected. In fact, they had never done it. But the foolish man believed. However, he did not give up his mind. He announced:"if I visit the princess in person, she will say yes." If he did so, you can guess the result.
Later, people use it to describe the man who only considers his own aspiration, ignoring the objective situation and others' opinions.