The young man saw, and without hesitation, walk to the right-handed door and opened it. He was pretty sure that the princess knows where the right door is, however, she didn’t. Each of them has different opinion. She wanted him to marry another women instead of dying, but the young man preferred dying instead of living without the princess.
In this situation the king was thinking if the young man really loves her daughter or just because she is the king's daughter. That was his point.
Before the young man opened the door, the king asked him: What do you want to be behind the door? The young man did not answer for a while then he said: If I have to choose, I want to die and not to marry another woman,no matter what is behind the door, I will love you daughter forever. The king was impressed by his answer because he was very honestand brave.
After that the young man was walking closely to the door. He was very calm and relaxed, however, the princess was extremely anxious and worried about the fate of her love. When he opened the door there was nothing behind it. He just saw a letter and it was written by the king. In the letter the king wrote:
Now I'm sure that you are really love my daughter, and you will sacrifice yourself for love. I can trust you to become part of our family.
Finally, the young man married the princess and they lived happily ever after.