Why did Aizen kidnap Orihime in Bleach? I remember that Aizen kidnapped Orihime but I don't remember if it was explained well why this happened. I have several hypothesis but not sure if they are true:


Was it that the Hogyoku didn't work until Orihime cured it some how? Was there such a scene in Bleach that gives evidence to this? Why wouldn't the Hohyoku work? Wasn't Aizen already giving more power to his hollows even before Orihime? 
Is that why Aizen needed her or was it something else? To Maybe torture her to distract the Shinigami and Ichigo? But it seems that at this point he didn't really thought Ichigo mattered so it doesn't seem convincing. 
Or did he use her to heal his Espadas? 
Or did he do that to lure the Shinigami and trap them to Hueco Mundo?


Is there maybe another reason? Or are there multiple reasons combined? Aizen was smart so I am sure he had a reason to kidnap Orihime.
The answer lies in Ichigo's existence. At the end of the battle between Aizen and Ichigo, Ichigo noted that Aizen's sword is filled with loneliness. Before Ichigo, there was no one of the same level of spiritual pressure as Aizen. This is proved in the conversation between Ichigo and his father, Kurosaki Isshin, on episode 301 of the anime.


   Isshin: Let's go (after Aizen)

 Ichigo: Don't you feel it? There's no way we can fight such monstrous spiritual pressure.

 Isshin: As I thought, you really can comprehend his powers.


That wasn't the exact words but is pretty much what they said in that episode. And previously during the battle between Aizen vs Isshin, Yoruichi, and Urahara, Ishin said that when he slashed Aizen, he didn't feel anything. Aizen felt so empty. This is because Aizen's power is far above Isshin, that Isshin can no longer comprehend (Isshin also mentioned this to Urahara after he unleashed Getsuga Tensho towards Aizen). This is later proven by Aizen when he made a normal human vanish just by being near him without the man actually understand that going closer to Aizen will kill him.

Why Aizen kidnap Orihime? The main reason is once again, Ichigo. Deep down, Aizen believed that Ichigo can reach his level. And to encourage Ichigo's growth, Aizen kidnapped Orihime so that Ichigo would feel the urgency of getting stronger.

The other, less significant (from Aizen's point of view) reason is Orihime's ability, Shunshun Rikka, which can negates reality. Aizen was curious about it when he saw Orihime used it to restore wounds.

As for Ichigo saying Aizen's sword is filled with loneliness, it was mentioned in episode 310 at 8:30-10:30 of the anime.

As for Yamamoto, he certainly was regarded as the most powerful Shinigami, but Aizen doesn't seems like seeing him as his equal. This is my own guess though.