I too suffer from re-startitus. Thank you for giving our condition a name.
I often restart because I wasn't fully committed to the story that was forming in my mind as I created a new sim or gave my simself a makeover for that save. When that happens, I know re-startitus is coming because I can't make up my mind what season to even begin in. Then there's the indecision about what world or what lot to start in. That's another sure sign re-startitus is coming.
Going back to my first point, not being committed the story, that is usually because I can't decide what career I want that particular sim to go for. I may start out wanting them to be...a scientist, but then I get the save started and suddenly think no, I want them to be a writer instead. No problem switch careers right? Not for me, because once I start rethinking that, then I start rethinking every other detail, and that brings my OCD into it...and re-startitus. When my OCD gets involved I will usually find something I don't like, normally it's something about the lot I don't like. It just spirals downward from there if I don't just start over from scratch.