If it's not something you feel excited for, then you definitely don't have to write it. I've found the stories that flow the smoothest are the ones you're deeply invested in telling.

That said, the excitement can come late. I'm not actually sure if my story will be NaNo ready by the time November rolls around, so I'm honestly just prepping it to help it take shape. Despite that, it wasn't really coming together. I feel like the recent prep assignments helped me make a bit of a breakthrough and give the story more substance, though, so the excitement is a bit more real to me now. Maybe not enough to make it NaNo ready, but it doesn't necessarily have to be. I'm just having fun with it, right now.

That said, if another story is begging to get out, I don't see why you shouldn't Prep it! You can write either of these stories anytime.