From a professional standpoint, it makes more sense to come to the Titans than to stay at Manly. He's already won a premiership with Manly, he's played Origin and been a national rep player. If he stays with Manly, he's replowing ground that's already been broken. But if he comes to the Titans, he has the chance to do something akin to catching lightning in a bottle, i.e., take a 'loser' club and turn them into a premiership-contending side. (I say 'loser,' because that's how every other team's supporters see the Titans...I know full well the exciting potential that is actually in our side.)
Think about it, if he could take the Titans to the top, it would become the defining moment of his sporting career. Who cares if you won a premiership with an already-dominant team? Manly's been to the top numerous times before DCE, and they'll be there again numerous times without him. But to take a team of cellar dwellers to the top of the ladder? If you toss that opportunity aside, you're a moron and/or you have a moronic manager.
Even if he doesn't win a premiership with us (or even make it to a Grand Final), he'll still be millionaire who already has a premiership under his belt and who plays at Origin and National rep level.