When Jack was trying to save Boone, Sun told Jack just to let go and Jack said "Don't tell me what I can't do"... When John Locke tried going on that walkabout samething was said to Locke and Locke said "Don't tell me what I can't do". How weird is that these would come clashing together one day.

This is a prominent catch phrase of Lost that fits into the Free will vs Fate theme.

Locke said this to his boss at the box company, to the travel agent in Australia and to Kate when he went after the boar. (Walkabout) He aslo said it to his teacher when he wanted to send him to a science camp in Cabin Fever. In Deus Ex Machina he said it to Boone. Locke said it to Jack in The Hunting Party.

Jack said it to Sun in Do no Harm, like you stated above. He said it to Des in MOSMOF and to Locke in Orientation.

But it is not only these two who have said it:
Claire to Jack - Raised by another
Claire to Charlie and Sayid - Exodus

Some others said it to Locke as well:
Ecko to Locke - LTDA
Charlie to Locke - Further Instructions
Ben to Locke - The Brig (This was my favourite one, Ben always has a plan and Locke could not grasp it)

Great recap, boncam! I remember those moments in vivid colour...

I personally love this line (as you can see in my sig!) It's become sort of a catch phrase for LOST in my warped head, tbh. :D

Thanks evanesco75 :D
I noticed it when I rewatched the show during the hiatus. Its a easy line to pick up.