User blog comment:Soulreaper1234/Why does Urahara Kisuke hid the Hogyoku inside Rukia's soul?/@comment-2038785-20100511203119

I think it was because he knew that Aizen was going to get it and it was fate or something and it was all part of their fate intertwining like the first searies of heroes with the timeline and shiz.

Anyway Urahara did say when Aizen was talking about how he couldn't control it properly and urahara comes back with "Yeah, you're right but only if you're on about the past".

That just gave me adleast the indication that it was like a plan against aizen. I mean he's fused (or "subdued" the hoyuku) how perfect would it be if that would kill him instead of making him all powerful.