Shane was already irrational, so having it all just be jealousy doesn't add anything or more importantly, do anything for the plot going forward.
I'm already thinking ahead to what they're going to do after S3, and we've done the prison thing, and having these characters fight each other and try to survive and not turn into monsters starts to get a little dull. Which it will after another 16 episodes of the same. It's fun now, but it can't last.
They need to keep ratcheting up the threat to hold our interest year after year. We've already seen how antsy people get when the narrative slows down like it did in the first half of this season. The ratings started to sag around that time, and then picked up as the action picked up, which is a big warning sign to the writers that they cannot let up the tension.
Yet they can't have a show that is only zombie-fighting and people fighting among each other. There's got to be some larger mystery that gets doled out judiciously, to keep the audience engaged.
They've got two ways to do this: the helicopter mystery (is there some form of civilized government still?) and curing the virus (there's gotta be some cure, or they're all doomed and then how does the show end?) Even if the cure fails, there's gotta be a plotline about it, so that we can have more episodes and keep the audience engaged for another year or two. AMC has got a golden goose and the show needs to use every trick in its bag to keep going and keep the story fresh.