as for why des and penny would be connected beyond sayid and nadia, at least in an island sense... penny continued to haunt and hunt for desmond while he was on the island, with her being desmond's sole purpose for moving on. sayid eventually got over nadia and fell for shannon.
Sayid knew Shannon for something like two weeks in island time. He pined for Nadia before getting to the island. He pined for Nadia while on the island and, later, married her after the Oceanic Six escaped. She didn't die until sometime after that. Sayid was so devastated he took Ben up on his offer to become Ben's personal assassin in the war against Widdemore.
With all that, we're supposed to believe his constant/soul mate was Shannon and not Nadia?
