Why does Hamlet finally take his revenge, after delaying for five acts? What has changed in him or in the circumstances?

Hamlet has his own existential struggle throughout the whole play that delays any sort of direct revenge. He muses about the nature of life and death. He questions the ghost being authentic. He berates his own courage. I think the death of Ophelia is a catalyst for his action. Fortinbras is also a character foil that spurs Hamlet to action.