Maybe because he gets enjoyment out of letting others struggle? I don't know, he did that quite alot. In fact it seemed to me that he didn't even do all this for the power alone, but to see other people suffer. Aizen even smiled at the Tousen blood explosion and Komamura going into rage mode because of it. He's that kinda psycho. Well and he also said it in this episode. He was happy that there was someone who could withstand him in some matters, so he could try out his new powers without people instantly vanishing.
Or possibly it's just...
Anyway, I wouldn't expect too much out of Bleach. It's Bleach.