Well, why did he?
He did it for strength and martial ability, not religious or philosophical reasons.
The Life of O-Sensei, Morihei Ueshiba
by Kisshomaru Ueshiba, Second Doshu
[Editor's Note: O-Sensei's son, Kisshomaru Ueshiba, recounted O-Sensei's life story, in his first book Aikido, which was one of the first books on Aikido translated into English. This is the first installment. Photos courtesy of the Aikikai Foundation from the book, The 100th Anniversary of the Birth of Ueshiba Morihei, Founder of Aikido Memorial Photo Collection.]
......Until he was 14 or 15 years old, the Founder looked quite weak with his short, thin body, but he was strong and his behavior was quite different from others. He had already had a general interest in budo since he was around ten.
When he was just twelve, his father, Yoroku, a member of the local council, was the main caretaker of the village. The so-called Toughs of the town, the hoods of his father's political opponents, would come to his house to negotiate. Sometimes they would rough up his father quite severely. The Founder said that seeing this happen so frequently seared a deep sentiment into his mind. He swore to become strong no matter what it would take, and throw out his father's attackers.
Young Days as a Soldier
In 1901 when he was 18, the Founder took the first steps in the direction of achieving his driving ambition
. He had come to Tokyo because he wanted to be a great merchant. He spent busy days working on a wholesale street, and studied jujutsu of the Kito Ryu at night. Some- times he went to hear political speeches, as well. However, within a few months he developed heart beriberi and had to return home.
On this occasion, he made up his mind to build a strong body and after recovering began walking two and a half miles every day. This continued for ten days. Then twenty. Eventually he began running. He slowly gained physical strength and became capable of lifting two straw bales of rice, while previously he had not been able to lift even one. By the time he was
about twenty he began to look quite different.
Although he was still short his body was much stronger than ordinary people's. But the Founder was not satisfied only to be strong. He went to Sakai to study Yagyu-Ryu jujutsu. During this time he was involved in fishery and boundary problems of his village, and helped in solving them. Through this work he became well known locally. It was also about this time that he became involved in so many activities that more than once he was a headache for his father.
The Founder was full of youthful vigor. He had an unyielding spirit. If others did twice as much as ordinary people, he would do four times. If others carried 80 pounds, he would carry 160 pounds. His quick temper found good opportunities for expression in the rice-cake-making contests of his village.
In these contests a large scoop of a special type of cooked rice is placed in a huge stone mortar or bowl. Then a large sledge, something like a wooden mallet with elongated head, is used to pound the cooked rice. An assistant constantly turns the rice over on itself as it is being pounded. Gradually the rice is transformed into a rubber-like substance which is laid out in flat cakes to cool before eating. The weight of the sledge with its awkwardly-shaped elongated head, and the force and frequency of the kneading means that making the cake requires a great deal of strength.
In these contests the Founder eagerly matched himself against other strong young men-four, six. Then ten. All were defeated. Finally the Founder broke the pounder. He would go to other places to pound rice and again broke many pounders. People eventually had to politely refuse the Founder's offer to help make rice cakes for fear he might break more of them. Instead, they served tea and pastries, in the Japanese way reserved for honored guests, to keep him away from the rice-cake-making area.
taken from http://aikidoonline.com/Archives/200...500_dosh1.html