Why does Hamlet ask Horatio to absent himself from felicity awhile?

It's act 5, scene 2, and Hamlet is dying. His dear friend Horatio, overcome with grief, picks up the poisoned goblet and prepares to drink from it. He'd much rather die the noble death of a Roman than live as a corrupt modern Dane. He wants to follow his friend into the next world. But with almost his last breath, Hamlet urges him not to take that fatal last step:

As thou'rt a man, Give me the cup. Let go! By heaven, I’ll have ’t.

Hamlet won't let Horatio die; he needs him to live on and set the record straight about his life:

If thou didst ever hold me in thy heart Absent thee from felicity a while, And in this harsh world draw thy breath in pain To tell my story.

On the brink of death himself, Hamlet concedes that to die would bring felicity, or intense happiness for Horatio. Yet he's keen to deprive Horatio of that particular happiness in order that the truth about Hamlet can be told and handed on to successive generations. Hamlet's urging Horatio not to commit suicide, then, could be seen as both a selfish and a selfless act on his part.