Returning to the castle had been a blessing he never expected. He had not wanted to return. Ever. Why return to the place that was the literal setting for almost every nightmare that tormented him? He had not returned when the memorial was held. He had not returned upon the re-opening of the school. Nor the Ministry Ball commemorating one year since the end of the war. Not for one of the many invitations extended by former colleagues to have tea or an after dinner drink. He definitely did not return when word leaked that the Interim Headmistress McGonagall could not gain access to the Headmaster's office and suites. (Minerva swore vehemently she was a place holder and had no interest in the job long term.). No one had been able to access them since that final, horrible day when Voldemort was defeated.
Though he didn't return, he did correspond with many of the staff. Apologies had been made, weepy friends and fellow order members appearing upon his threshold begging forgiveness for their stupidity and folly. What could he do but acquiesce? In his mind, there was nothing to forgive. These people had believed what he and Albus had wanted them to believe. It was the only way their plan would have worked. And, they were the closest he had to family. So he offered them the absolution they sought and asked them never to speak of it again.
He tried not to think of Hogwarts, too many conflicting emotions, but in spite of himself he was intrigued by the stories of unusual behaviors by the castle itself. While the structure itself had rebuilt the areas damaged during the Final Battle, it was not the same. The inability to gain entrance to the Headmaster's office was just one quirk. Certain areas of the castle could not be illuminated, not by magic or sun shining through the windows in the area. The staircases seemed to take a malicious joy in sending Board of Trustee members and Ministry officials on an infinite loop until they realized they were literally getting nowhere and departed in a huff. The house elves were sullen and though they carried on their work as before, they muttered about how the castle was not happy and it would not be a good place to live until it was happy again. When asked to elaborate, they only replied with a variation of the same answer, "That's the castle's business." They would say no more.
And the animals of the Forbidden Forest, that was another thing. No birds flew on the grounds, no small wildlife were seen, even the Great Squid had not surfaced and some feared he had been another war casualty. But at night, the sounds from the Forest were unsettling. Mournful cries of all manner of creatures filled the evening with a dreadful sound that filled listeners with sorrow. There was no explaining it but everyone sensed this was another byproduct of the castle's surly mood.
Magical architects and engineers, seers, shaman, even a Headmaster from America, were all brought in to try to find the source but to no avail. They all sensed something was terribly wrong but it was beyond their powers to pinpoint the source, much less fix it. Hogwarts was not like other buildings and so could not be repaired as such.
Yes, the school was reopened and the students were safe but there was no denying it was a different place than before and not because of any events that had happened during the war. No, everyone could tell, the castle was unhappy, as fussy as a baby wanting a bottle. If only the solution were that simple.
In a last ditch effort to rectify the situation, Minerva showed up on Severus' doorstep asking him to come back and have a look. Just to give his thoughts. Nothing more. He wasn't sure he believed her, she'd been trying to convince him to accept a post there since before he was even healed. But she insisted that things were getting worse, more rooms were locking themselves and unable to be opened, entire hallways had disappeared behind stone barriers. As the only living former Headmaster (she hadn't meant that to sting and yet it had), couldn't he return and assess the situation? He had previously had a bond with the school, maybe he could interpret what it wanted, what it needed. So he agreed. She was a tenacious witch and he felt he owed it to the castle to try if nothing more.
It had been rather simple really. What it wanted was him and it was immediately apparent they both needed each other. He knew this truth the moment he crossed through the gates. Every fear he'd had about returning was wiped away once he was on the grounds. And upon entering the castle, an effervescence bubbled through him seeming to cleanse the cobwebs in his mind, the terrors of the past, the aches in his body. The castle responded in kind, putting on a display of playfulness and joy culminating in the staircase taking Severus and the delegation of officials in attendance straight to the entrance of the Headmasters office. When the doors opened, no one was allowed through until Severus had entered. Not a word was said but a decision had been reached. Hogwarts had found its Headmaster.