What is the main reason why Yan Zhenqing can become a calligrapher?

During the Anshi Rebellion in the Tang Dynasty, Yan Zhenqing and his cousin Yan Gaoqing held a banner of righteousness to fight against the rebels. Gao Qing and his son died heroically. When Yan Zhenqing drafted his eulogy with running script, he was full of emotion and indignation. He is "angry but dangerous in his words" and "depressed in his words". Mo Miao understood this spirit, was emotional, and expressed the author's voice.