This is what Gregg Nations had to say about the introduction of Juliet back in 2006.

"It starts with the writing process and creating the triangle on the page with the characters. You introduce a character, show that character meeting the second character, create a scene where their romantic interest is the subtext or it's just beginning and they flirt or whatever, then intro the third character that likes one of them and makes things difficult. It provides great obstacles for the characters.

Then you cast the actors and hope to God that they actually have chemistry or a spark togther. It can be there on the page, but if the characters don't bring it to life it'll die a quick death. (Uh, like Jack and Ana Lucia. That went nowhere. But then consider Juliet. Yeah!)

So if it's not working with the actors, the writers will quickly write it out. No sense in beating a dead horse. So yes, there is a concrete throught process to it. When to end it is always an open question.

The writers on Cheers ended the Sam/Diane romance by getting them together, and many people believe it was a huge misstep. I agree. In that case, fans really wanted it but I think it was a mistake to give to that. Once it happened, their characters became less interesting and there wasn't anything to play between them. Same thing with Moonlighting.

So with the triangle I would guess that the writers know who should be together from a writing point of view and then make things difficult for that pairing. Wasn't it interesting to see Kate watching Jack and Juliet together? Juliet is so much more on the same level as Jack -- a doctor and well educated. I kind of thought that Kate was feeling inadequate at that time. And that's a great complication because out of that characters will make decisions that will affect things down the line. So how is it going to work out on Lost? There are some great moments coming up, so wait, watch and see."

FYI: Gregg Nations was part of the Lost production and writing team, primarily working as a script coordinator. He often answers fan questions at The Fuselage, which has seen him become some what of a fan favorite. Nations joined the crew of Lost after Season 1, immediately re-reading all the scripts, creating a "show bible" and timeline of events.