"It was unbearable, he would not think about it, he could not stand it... there was a terrible hollow inside him he did not want to feel or examine, a dark hole where Sirius had been, where Sirius had vanished. He did not want to have to be alone with that great, silent space, he could not stand it--" - OoP37
The Five of Cups is a warning of loss and grief. There will undoubtedly be feelings of regret and denial associated with this card, but it also promotes change and moving on, instead of stubbornly wallowing in the pain.
The death of Sirius comes as a devastating blow to Harry, who is forced to face the loss of the closest person he had to the family he so much craved. It is important for him to learn, however, that he can't get endlessly hung up over the tragedy like he did after Cedric's death, but must face the pain in a mature manner in order to heal.
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