when he was the Juan Lian Jiang Jun, accidentally broke a glass lamp during the Empress's Peach Banquet. He was given 800 strokes and banished to Earth by the Jade Emperor. All because of a single glass lamp.
Wu Jing was changed into a demon, and once every week, flying swords were sent down from Heaven to penetrate his chest a few hundred times over. He had nothing to eat, so every few days he had to go out of the river to eat a passerby. One day he attacked the Goddess Guanyin by accident and she advised him to redeem his sins by becoming San Zang's disciple and protecting him throughout his journey to the west.
So he bided his time in the River of Flowing Sand until San Zang, Monkey King and Pigsy finally came along. Not knowing that they were the pilgrims, they fought and in the end because he couldn't fight in water Wu Kong had to ask for help from Guanyin. When Wu Jing realized that they were the pilgrims, he surrendered immediately, became San Zang's disciple, and joined them on their journey to the West.
he has a bland, pale, tasteless personality compared to his other two brothers. So, Sandy can be seen as the glue holding the group together, the good guy, the ever-dedicated one, the patient one, the baggage-carrier. Basically, he is your average, normal, nice guy... as normal as a water demon turned Buddhist monk can be.
Photo credits: Shupian.