When Harry was eleven and looking into the Mirror of Erised, Dumbledore finds him and, when asked what he finds in the mirror, says that he sees himself holding socks, because everyone insists on giving him books.
I think Dumbledore's power and wisdom have made him a slave to himself. As the books have gone on, he showed more and more signs of the pressure and stress of that slavery. Because socks are a sort of symbol for freedom in the books (think Dobby), I think he sees himself being free to live without the feeling of having to care for everyone.
The biggest difference between Dumbledore's death and Sirius's death was that Sirius died suddenly, somewhat suprised. It was a total shock, not just to us, but to him (remember that Rowling describes the look on his face as shocked). Dumbledore knew he was about to die, and I think he almost saw it as a sweet release from the pressure he was under.
I've got to say, while I believe Dumbledore saw something alluding to freedom in the Mirror of Erised, I don't think he actually saw himself holding socks. Is it possible he could have seen something as morbid as his death as his greatest desire? It's almost as if he knew Harry had the information and resources he'd need to do the job he needed to do, and he probably knew that the only way for him to be without the slavery and pressure of respect bordering on reverence was for him to die....
Or maybe I'm just morbid.
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