You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Martin_Luther

Context:
He later attributed his decision to an event: on 2 July 1505, he was returning to university on horseback after a trip home. During a thunderstorm, a lightning bolt struck near him. Later telling his father he was terrified of death and divine judgment, he cried out, "Help! Saint Anna, I will become a monk!" He came to view his cry for help as a vow he could never break. He left law school, sold his books, and entered a closed Augustinian cloister in Erfurt on 17 July 1505. One friend blamed the decision on Luther's sadness over the deaths of two friends. Luther himself seemed saddened by the move. Those who attended a farewell supper walked him to the door of the Black Cloister. "This day you see me, and then, not ever again," he said. His father was furious over what he saw as a waste of Luther's education.

Question:
On what did Luther's friend blame his sadness and entrance into the cloister?

deaths of two friends