Dumbledore was a lonely soul after Grindelwald. No one had managed to get closer than a friend. The number of people he would consider close friends could be counted on a single hand with fingers to spare.
Dumbledore didn’t want to be lonely it was simply that he found himself rather busy or that others had a hard time looking past the tale of the Defeater of Grindelwald. He had in fact always wanted a child to fully pass on his knowledge and ideals. To know his legacy would live on. Of course, due to his preferences when it came to love, the child would need to be adopted, and he had made peace with this.
Just such an opportunity presented itself one Halloween night.
Dumbledore decided to take in Harry and raise him as his own. It couldn’t be that hard right… right?! The boy would have a tough life and Dumbledore could raise him to handle his fame and power.
Dumbledore needed help. He didn’t know how to raise a child. He also worked too many jobs ugh. He couldn’t go to Minnie; she told him it was a bad idea to begin with, and then she would never let it go. Sucking up his pride, he brought the child to the staff table during winter break and asked for everyone’s help.
This is the story of why Harry calls all the professors Aunt or Uncle and why he calls the big man himself Grandfather.