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[Harry Potter] Why does the mind connection between Harry and Voldemort reopen in the Deathly Hallows?

In Order of the Phoenix and before, Harry experiences pain in his scar and visions of Voldemort and his mind when he is feeling a strong emotion. Eventually, however Voldemort becomes aware of this and uses it to deceive Harry. But, he begins practicing Occulmency against the connection after possessing Harry and suffering extreme pain. All throughout Half-Blood Prince, the connection remains closed.

Given this, why does the connection reopen? Does Voldemort believe he can abandon caution due to Dumbledore’s death? Is Voldemort aware of the reopening of the connection? It seems like if he was aware, he would probably use try to use it to his advantage again, or at least try and stop Harry from seeing his actions to find the Elder Wand or secure the Horcruxes (especially since he knows that Harry’s onto him...).

More generally, why does the frequency of the visions increase in Book 7? Does Voldemort ever randomly get flashes of Harry’s mind? In Book 5, Voldemort was aware of Harry after he witnessed the snake attack—why doesn’t a similar thing happen in Book 7?
gpt
In Harry's fifth year, Voldemort is weaponizing their connection to plant visions in Harry's head, and Harry lacks the means of defending against them for most of the year, culminating in Voldemort attempting to possess Harry's body and goad Dumbledore into killing them both.  However, right before that final possession, the loss of Sirius gave Harry the emotion he needed to secure his form of Occlumency; Dumbledore called it love while Harry called it grief when he became conscious of it two years later after Dobby's death.  Attempting Legilimency against Harry while he's emoting love/grief is painful to Voldemort, and so Voldemort consciously blocked the connection.

The connection reopens because Voldemort's focus on maintaining the block slips in moments of high emotion on his own end, such as excitement in his pursuit of the Elder Wand and realizing Harry had fallen for the Nagini trap, and fury and fear when he realized Dumbledore and Harry had discovered his Horcruxes.  On Harry's end, he allows the connection for insight into Voldemort's actions despite Hermione guilt-tripping him for it.  

Voldemort does not seem to be aware of the reopening, especially because it unconsciously betrays the existence of the Diadem within Hogwarts to Harry.  Even if Voldemort knew of the reopening of the connection, he wouldn't try to possess Harry with it again since Harry could just smack him with Occlumency emotion at that point if he really wanted to.

The frequency increases due to the increasing number of events that excite or frighten Voldemort (Elder Wand pursuit, opportunities/missed chances to kill Harry, discovery of the Horcruxes being hunted), making him mentally vulnerable.  It's unlikely that Voldemort gets flashes of Harry's mind at that point since his goal is no longer to probe Harry directly, and if he did he would have known of Harry's location(s) and the Horcrux hunt much sooner that year.