Minerva McGonagall is often worried that she will not be able to do enough for Hogwarts. Harry often discusses this with her in the office that she feels unworthy of. The first time that this discussion occurred, Harry had just given a History of Magic lecture and Minerva had called him in to discuss what he had done well and how he could improve.
But Harry had immediately noticed something wrong and asked her about it. Surprisingly, Minerva quickly admitted to feeling overwhelmed and incapable. Harry did everything in his power to convince her that she was quite capable, but she simply remained in despair.
That is, until the spirit of Albus Dumbledore burst into the room and enveloped Minerva in a comforting embrace.
For a split second – only a moment – all of the worries of Minerva's life were gone and Harry noticed that her blissful face looked years younger.
Dumbledore's spirit let go of Minerva, but Harry felt it flowing throughout the room, sticking near her. At this point, Minerva was smiling slightly, and Harry was able to talk her out of her misery. When Harry left the cheery Headmistress behind, he felt the spirit of Dumbledore floating beside him.
"Thank you, sir," he said quietly, smiling. Dumbledore's spirit swirled around Harry once before slowly fading away.
But whenever Minerva McGonagall was in despair, particularly about Hogwarts School of Witchcraft and Wizardry, Albus Dumbledore's spirit would be there to lift her up.