He hurt her in his previous life; when he met her in his future life, he fell in love with her hopelessly.
In her previous life, she loved him to death, and that didn't change until the last moment of her death.
She, a beautiful girl in the 21st century, traveled through time and space because of Yue Lao Niang's prank.
He, with his superb medical skills, saved her from dying. When he thought she was dead, and after he changed her appearance, she came back through it.