When Harry Potter learned the truth of his magical abilities, he was thrown head first into a world of mystery and history that further informed how integral his existence was. As a result, while he was the most important wizard by fate, history has proven that the real crucial character of the series has been none other than Albus Dumbledore. From the time of Grindelwald to the threats of Lord Voldemort, Dumbledore had always been there, ready to make the necessary sacrifices and always seemingly one step ahead.
While Dumbledore evoked a sense of bravery and strength, he still made mistakes in his life. But unlike Harry and his friends, who made their own mistakes in their life, Dumbledore's were mistakes that haunted him for the rest of his life. Moreover, some of these choices likely made him more cold and calculating than when he was younger. But according to Reddit theorist HuntingTheWumpus, Dumbledore's cause for his recklessness and coldness had to do with his biggest secret of all.
Dumbledore Was a Time-Traveler
In Harry Potter, Dumbledore spent most of his life making up for the biggest regret he had: dealing with the death of his sister. Her loss pushed him to continue learning magic, becoming a teacher, and being proactive in righting the wrongs around him. A great example of this was his feud with Grindelwald, where, once their blood pact was broken, he finally could defend against the dangerous wizard. But his actions could've also been fueled by a gift he gained where he could revisit moments thanks to a Time-Turner.
Time-Turners allowed the user to venture back or forward in time and potentially influence events that happened in the present. This was how Harry saved himself from the Dementors in Harry Potter and the Prisoner of Azkaban. However, Dumbledore likely used his ability to travel to initially try and right the mistakes of his past, like saving his sister. But his failure at every attempt could've shown him that changing time isn't possible, but influencing events could make a difference. As a result, he strived to find other ways to gain closure over his sister and focused more on putting himself and his loved ones in a position for success against enemies. But this still changed him.
How Did Time-Traveling Affect Dumbledore's Personality?
When Hermione used the Time-Turner in Harry Potter, Dumbledore warned that terrible things could happen to a wizard who messes with time. In his case, he likely meddled in his own affairs too much, influencing events against Grindelwald all the way to Harry Potter. This also caused him to see his own death, meaning he was even more jaded, as he knew he was nothing more than a pawn in a larger scheme. Therefore, he focused more on ensuring the timeline continued, and he, overlapping with other time-traveling versions of himself, moved the pieces across the board to influence those around him.
In doing so, Dumbledore likely also lost himself and a degree of his humanity as he saw the facts of life in a way no witch or wizard ever should. He grew colder and, more often than not, forgot that he was still one that needed to impact the students around him. in the end, he knew the importance of his sacrifice and would never stop trying to help his sister. This may have helped him against the final battle with Voldemort, but it also made him a reckless wizard, and his escapades through time may also explain why he seemed so all-knowing and was always right where he needed to be.