For all you literature geniuses, we are currently pondering in class (1/2 literature) the impetus behind Hamlet's change in behaviour and mindst at Act 5 in the closing scenes of the play.
We are trying to pinpoint what actually causes it. So the ever present question WHY??
I am kind of lost as to answering this question at the moment. All I have come up with is that Hamlet has had time to heal and to ponder- so the grief which affects him in the earlier acts of the play has less of an effect in Act 5.
If and of you are currently doing Hamlet or have studied in the past, any thoughts would be absolutely fantastic! 
I would actually identify Hamlet's turning point at Act 4 Scene 4 and his " How all occasions do inform against me" soliloquy. If you analyse this scene in a close reading you very quickly realise that this is the moment when Hamlet gathers his resolve. Essentially this is the epiphanic moment when he aligns "word" to deed and realises the massive power that can be wrought by something as passive as thought. Take notice of his discussion of Fortinbras' army, " of such mass and charge, led by a delicate and tender prince." He's drawing a comparison to his own circumstances: he acknowledges that while the abstraction of thought is in itself ineffective, through action thought has powerful outcome. He thus fulfils these Enlightenment ideals surrounding the archetypal man as an amalgamation of both mind and body. ( see his "what a piece of work is a man") Did you notice how throughout the play, Hamlet either traps himself within introspection which prevents him from taking revenge, or else his actions are rash and unpremeditated? Well in the " How do all occasions" soliloquy Hamlet achieves equilibrium between these two elements of human existence. That's why the next time we see him in the graveyard scene Act 5 he claims his identity as " Hamlet the Dane. " After spending so much time grappling with the dichotomy within his mind " a thought quartered" and struggling with an uncertain identity, here he is able to confidently proclaim himself. In Act 3 Scene 1 he questions whether " to be or not to be"; in Act 5 scene 2 he simply says " let be". His existential anxiety is gone as he comes to terms with himself, mortality, and the chaos of a Denmark which stews in corruption and despair. Effectively he acquiesces to fate, essentially saying "let all those things be; let life or death simply be; I will accept whatever comes to pass." His metaphysical and physical quest ends.
Wew! It's been a while since I talked about the Ham man. Hope that helped a bit.