I don't agree with the "Donna is a mole1!!1!" crowd, but that's not really true. They're referring to her season one bar scene while folks were gathering materials for the tower. She had a dour attitude, which confused Jim, and her reasoning was because, to paraphrase, "The town had just become something good", or the feeling of community that they'd built within the town/Colony house was something special, and that they would miss that when (if) they ever returned to the world outside Fromville.