The very nature of training "aiki" is hard, repetitive, challenging, and I think requires single-minded dedication. Not everyone will ever want to put in the work. Not everyone will take it as far as they can. But, by many accounts, Ueshiba seems to have done all that.
That dedication requires a certain character trait. It's that character trait in Ueshiba that I think drove him to Deguchi. I'd guess that there were Shinto priests around Japan that Ueshiba could have gone to and learned from. Had Ueshiba truly wanted to delve into being a Shinto priest, I doubt that much would have stopped him. So, I'd guess that there was something in Deguchi that appealed to Ueshiba.
I'm going to quote a post in another thread
Don J. Modesto
3) Omoto as Shinto.
Shinto underwent a decided mongrelization coming into Meiji, ironic as Tokugawa just before it was a period of intended purification of "foreign elements" (Buddhism). Trembling at the impending threat of colonization by Europeans, foretold by Perry's hubristic gallivanting into Uraga Bay, the Japanese wanted unity to repel the foe and used Shinto as a means to this end. Having tried to destroy Buddhism (many Buddhist monks simply shed their monastic robes and put on priest's clothes), the authorities demanded that religious expression fall within the purview of State Shinto, ie, join a shrine system or disband. Deguchi had actually gone to Shinto school (for 2 years, IIRC) and so qualified. But his actual practice was more in line with shamanism than the ritual of Shinto, and could be considered Shinto mainly through political posturing.
A post to archive on the subject, is Wm Bodiford's at: http://www.e-budo.com/forum/showpost...9&postcount=18
Don Modesto makes a comparison to what Deguchi was doing to shamanism. From light research, I'd guess Omoto was definitely different than most Shinto practices.
Something in those two differences (Deguchi himself and the Omoto), either individual or together, caught Ueshiba's eye. If I had to guess, I'd say it was Deguchi himself and not the overall Omoto.
Here's the leap ... we've all heard about people who have had experiences that are outside the range of "normal". From people who say they were saved by their guardian angel to those who see ghosts. What if Ueshiba had those experiences when younger? Or had a few of them throughout his life? What if he heard that Deguchi had similar experiences? I think Ueshiba would have studied with Deguchi with as much fervor as he had with Takeda.
Maybe in Ueshiba, you had a man who was guided by the kami from early on. Maybe he wished for things and was granted them. Or perhaps he really was guided by some unseen force and he knew it. Is it a stretch to imagine Ueshiba having a few "weird" experiences and wanting to have more, finding Deguchi to have had similar ones (or perhaps not, maybe Deguchi was just stringing him along ... who knows. Deguchi himself was abnormal), and then learning Omoto to try to activate those experiences rather than wait for them to happen again.
We do know that Ueshiba was drawn by two very different worlds: martial and spiritual. He dedicated a large portion of his life to both. When he found what he was looking for, he threw himself into it with dedication.
He seems to have found what he was looking for in Takeda and Deguchi. And with the nature of aiki, Ueshiba found (pun intended) a system for merging the two.
I find it hard to believe that he was a religious nutcase. I find it more likely that he had at least one significant spiritual experience that most would find hard to believe, that the experience changed his views on the world, and that he searched for the method or reason behind that unique spiritual experience.
Just as he found his martial teacher, so he found his spiritual one. Some people, I think, would personally understand just how that happened.
But fact or fiction? Spiritual experiences are a step beyond the martial. At least in the martial world, IHTBF is a rule. It only takes a few moments training with someone who has real "aiki" to understand the defining differences in martial skills. With the spiritual, IHTBF is useless. So what if you're training with someone who's saying they're an avatar of the kami. They either have aiki or they don't in the world of IHTBF. In the spiritual, IHTBE rules supreme. It Has To Be Experienced. It's why I mostly stay away from the spiritual side of Ueshiba and Aikido. Unless you have those defining spiritual experiences ... there's no understanding.