the prince's reason

Actually the prince left for a war - to protect the princess' kingdom from kinda bad guys.

Unfortunately the bad guys were too strong to be defeated, even the prince had put all of his hope and his effort... there he was, lying on the battle field, wounded so badly with the rest of his armies.

"Are you willing to die?"
"No. I still have to meet my fiancee. She's waiting. So please help me...", the prince begged the unknown man who came and stood nearby.
"I shall give you this for your heavily wounded body. Too broken to be mended," at one glance, he bit the prince.

"NOO! What am I going to say to her? It can't be like this way!"
"Yes, it can be. It have been this way," the unknown man soon left.

"What will she do if she know that her fiancee has become an immortal - a vampire ?"
Then in his frustration, he went from the battle field, ran away from her fiancee while seeking a way to be a human, once again.