'I just need to show them something'- Its about Love and Sacrifice by jumbotron007

I understand this theory is not complete but it looks like the way the show is heading toward. I m not sure if this has been posted before and if so my apologies.

What is Desmond gonna show the losties? Why did Desmond go with Sayid?

Its pretty clear now that all of them who have lost their love have found their redemption somehow or the other in the FST. and FST is the place where they want to be.

I think after Desmond met Penny ,his constant his consciousness merged and both the Desmonds became one.The FST Desmond knew about the losties and the island Desmond knew about the FST.( its similar to the scene in the constant when he says he feels perfect in the island and at the the other Desmond from 1996 smiling after getting Pennys number as if knew what was going to happen.
i think he will meet all the losties in the FST and also in in the island and convince the island losties that there is a beautiful world for them waiting and to get there they ll all have to'die'. All the dead people who have made a sacrifice have realised their dreams .For example Charlie has realized his dream but for claire too to be on the same line her consciousness from the island must travel to the FST. To make all the Losties want to transition from the island to the FST Desmond will have to collect some proof about what the life the losties are having in the FST and 'show them'- the island losties about what they ll get in the FST if they sacrifice here.Everyone has something to gain for- Sawyer can get to be with Juliet,Jack with Kate and/or be a good father,Hurley can be a lucky man,
Sayid can see Nadia etc.... But to Desmond it will mean giving up his life in the island timeline with Penny which he fought so hard to attain , but his consolation will be that he can get a second chance in the FST with Penny. His quest will be over. I think Widmore also has the same agenda.He wants to correct his past mistakes. He wants his son back and he wants Penny to like him.
This is the reason Desmond went with Sayid. He needs some convincing to do to make them sacrifice.
They all will have to work together to destroy the island.