In "The Canterville Ghost" by Oscar Wilde, why does Sir Simon become disappointed and start to think about his existence?

In Chapter Five of "The Canterville Ghost," the ghost admits to Virginia that he wants to enjoy eternal rest in the Garden of Death:

"For three hundred years I have not slept, and I am so tired."

The ghost's sudden interest in death is a result of two factors, the first of which is his treatment by the Otis family. Since taking possession of Canterville Chase, the family has thwarted his attempts to frighten them and reduced him to a state of depression, as described in Chapter Four:

His nerves were completely shattered, and he started at the slightest noise.

Secondly, as a result of his depressed mood, the ghost is no longer able to fulfil his supernatural obligations. As the ghost of Canterville Chase, for instance, it is Sir Simon's duty to frighten the family by appearing in the corridor every week and "gibbering" from the window twice each month. Suddenly unable to complete these tasks in earnest, Sir Simon experiences a crisis of identity. In his mind, the only solution to this problem is to die and be permanently freed from the Otis family and his ghostly duties.