luke is only there because he believes in destiny. in ESB, vader confirmed it was luke's destiny to destroy the emperor: "he has forseen it". this is why luke let go of the gantry: he knew he wouldn't die. how could he? he had a "destiny".
palpatine's taunt is the equivalent/parallel to obwan's line in ANH: if you strike me down I shall become more powerful that you can possibly imagine. (read: if you strike me down, I WIN) -- these were the ground-rules of the ANH duel, stated explicitly, as the duel begins.
(palpatine is merely stating the ground rules; the duel begins in the next scene
"if you strike me down I will become more powerful that you can possibly imagine")
luke believes it's his destiny to stop the emperor. he heard it from vader in ESB. the emperor has just told him, the only way that can happen, is if/when luke turns to the dark side. now luke is starting to beLIEve it. (why? because he believes in destiny -- the reason he came, is the reason he shouldn't have come -- and the duel commences).