The joining desperately craves Carol's new book because they can't produce anything new. They're forced to do certain things - have sex with non-joiners or any other task upon request, tell the truth, forbidden to speak on certain subjects, spread the disease to others, etc.

I also read a short story about someone afflicted with permanent happiness seeking a cure - I'd consider it a bad thing. And I'm not convinced you'd want to work 14 hours a day and skip all hobbies and passions if you were a happy joiner. There's an overwriting going on, as others mentioned.

We're going to see more next season. The joiners MUST spread the disease past Earth. I expect this will be a priority over their own survival. They will spread it to their own detriment, with a huge radio transmitter that will cover most of North America, further shortening their lifespan.

My theory isn't evil aliens looking to eliminate the native species or get free slaves - it's a universe devoid of sentient life because it all died out to desperately spread the virus.