It turned Mary Sue? Really? I would think that hometown stories would just turn into secret bitch-rants about people you know.

yeah. I think that's a saying fantasy writers in particular like to fall on. Of course, it might be really good not to take the easy way out and actually try to figure out those things, but yeah.

You say you prefer fantasy, but that little sci-fi example was kinda awesome! I think when you do have reasons, even fantasy based ones, it makes things more real to the readers. too much 'because I said so' requires a lot of suspension of disbelief and that would pull the reader out of the story. Being specific is important.