Briefly describe the causes of Tibetan opera stories with the following words.

It turned out that there were no bridges on the Yarlung Zangbo River, countless cowhide boats were overturned by rapids, and many people trying to cross the river were swallowed up by the roaring river. Therefore, the young monk Tang Dongjiebu made a great wish and vowed to build a bridge to benefit the people. Tang Dongjiebu, who had nothing, only got a burst of laughter.

So there is such a legend. Tang Dongjiebu met seven girls who can sing and dance in Qiongjie, Shannan, and formed a Tibetan opera team. He advised people to do good deeds, donate money and build bridges with fellow initiates in the form of performances. People are very happy. They walked from one bridge site to another ... the seeds of Tibetan opera are all over the snowy plateau. In this way, Tang Dongjiebu, penniless, left 58 cable bridges on the Yarlung Zangbo River and became the originator of Tibetan opera.