"..." He has a lot of questions about this but Jack is impatient to get to the wish so he figures he'll never know. The pirate might be beating him to death very soon. He considers putting on the skin and pretending to grant a wish. How would Jack know unless he tried it out? And if he did the pirate would be dead and unable to hurt him so...
"Alright. But first, a warning." He's keeping his back to Jack because he's just really unused to lying and afraid the pirate will see it on him like he had before. "Eternal life isn't without pain. You can still be hurt. Suffer wounds no man should live through. It cannot be taken back. Once I grant you this wish everyone around you will age and die while you go on. Your children.. your family.. your friends. You don't understand what you're asking, Jack. You don't know how hard it can be to outlive those you care for. To meet them as young women and return to them only to see an old crone bitterly looking out on the sea and wondering why you never came back when she was still beautiful. Men aren't meant to live forever."