IMHO I think part of the reason they showed the scene where Jack was using his father's prescription pad and said Have my father come down here if he isn't more drunk than me comment was to show how messed up he is after he is off the island.. Jack was all messed up on pills, drinking, and what not whose to say that he isn't suffering something similar to post traumatic stress syndrome.. Suffering from delusions and this is what the writers are trying to get across.. Kate seems to me to think he has gone off his rocker and I don’t think it is the first time Jack has contacted her like this.. What do you all think??

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Please don't give up, Des. Because all we really need to survive is one person who truly loves us. And you have her. I will wait for
you. Always. I love you, Pen

DESMOND: I love you, Penny. I've always loved you. I'm so sorry. I love you!
PENNY: I love you too.
DESMOND: I don't know where I am, but--
PENNY: I'll find you, Des--
DESMOND: --I promise--
PENNY: --no matter what--
DESMOND: --I'll come back to you--
PENNY: --I won't give up--
BOTH: I promise. I love you.