I'm the same with GSD's. I know many of them are beautiful and friendly but when I was younger I was at a family friends' house and their GSD chased me around the backyard until I climbed the wood pile where it couldn't reach me Since then I have always been a bit wary of them. I have also heard stories from an elderly family member who owned a GSD who bit her when she was feeding him. Also a bit wary of Rotties - not sure why though
ETA: having said this I do know many examples of both breeds who wouldn't hurt a fly, so I know every dog is different and you can't judge a whole breed based on one experience!!