Question

why did the wedding guest beat his breast?

Answer 1

He beat his breast because the loud bassoon broke the hypnotic spell the ancient mariner had cast upon him. He remembered he had to attend the wedding of his kin. He did not want to hear the tale of the mariner. So in irritation, he beat his breast.  

Answer 2

Explanation:

The wedding guest beat his breast because he had been held up by the mariner while he could hear the loud bassoon, which was an indicator that the bride had reached the hall.