Once Voldemort realised he could access Harry's mind in Harry Potter and the Order of the Phoenix, how did he not realise this was because part of his soul was latched onto him?
Because he was under impression that he is using magic for same. He was using Legilimency Spell for same, the part of his soul made the connection strong , however Voldemort would have been able to access Harry's mind anyways.
The Legilimency Spell (Legilimens) allows the caster to perform Legilimency; in other words, to delve into the mind of the victim, permitting the caster to see memories, emotions and thoughts. Skilled Legilimens, such as Lord Voldemort, are able to perform Legilimency without using the incantation or a wand.
Voldemort has used this spell extensively, both wandlessly and nonverbally, to enter the minds of those he wished to interrogate. Due to the link between himself and Harry, both can access each other's thoughts if Voldemort does not block it off with Occlumency.
Voldemort might had believed that Harry is weak and hence his mind is easily accessible.
Most of people believed this connection is due to the scar gave by Voldemort. He himself believed same, he never thought about soul being latched as this was rare case.
- No one knew this was possible – except for Professor Dumbledore himself.
- Voldemort’s huge ego prevented him from analyzing it further.
No one knew this was possible.
Up until it happened, everyone thought the Killing Curse could not be stopped or blocked. “Professor Moody” (Barty Crouch Jr) said so when he was teaching Defense Against the Dark Arts. He stated that the only one known to have survived this curse is Harry. And the reason for this was a mystery to all but a very few. Harry was simply “the boy who lived”.
Only Dumbledore (and later Voldemort) figured out that the Killing Curse could be made to rebound off of someone who is protected by the ancient magic of love. Lily Potter died to protect the son whom she loved. Only Dumbledore reasoned out the following:
- Because of Lily, Harry was protected.
- The Killing Curse rebounded and hit Voldemort, destroying his body but not his soul because of the horcruxes Voldemort created beforehand.
- Harry could speak to snakes.
- Only the rare descendants of Slytherin spoke to snakes.
- Harry was either a descendant or something else.
- An additional part of Voldemort’s soul was ripped from what remained in his body and latched onto the only living thing in the vicinity: Harry.
- That's why Harry can speak to snakes.
Again, only Dumbledore riddled this out (I guess the pun was intended). No one else knew this. Recall when Dumbledore was giving instructions to Professor Snape on what to do after Dumbledore died. He had to specifically say that only Voldemort is the one who had to “kill” Harry. He had to give these exact instructions because no one knew (including all the Death Eaters) that Harry had to have Voldemort’s soul fragment removed before Harry could be cured. So Harry had to be "killed" by the only person where the Killing Curse would not work -- Voldemort. And Voldemort's ego pretty much guaranteed that he would be the one to "kill" Harry.
Voldemort’s huge ego prevented him from analyzing it further.
Voldemort never took the time to be this analytical – because of his enormous ego. He was very intelligent, but not interested in being an academic. He just wanted to be the most at everything he considered important: power over everyone, eternal life, the elegance of his existence. When Voldemort realized he could reach Harry’s mind, it was easy for his ego to make the mistake of thinking his extraordinary talent at legilimency allowed him this unprecedented reach (probably boosted by using Harry’s blood to reanimate).
This is not the only time that Voldemort’s ego led him to make crucial errors in judgment. Snape used this to his advantage when his skill at occlumency allowed him to move within senior Death Eater circles as a spy. Voldemort never suspected that his prowess at legilimency could be bested.
Also please recall that, when Voldemort was disappointed while using the Elder Wand, he said that he did extraordinary things with it because he was extraordinary, and not because the Elder Wand had chosen him. That led Voldemort to incorrectly think that the Wand still belonged to Snape. So Snape had to die by his hand in order to have the Wand change allegiance. This mistake was a direct consequence of Voldemort’s unbridled ego.