Are you doing most of your research online? Using the "general description" that is usually spread on the internet about this line or that line? That could be a huge problem...
Don't take offense to this...but you have no idea what you're talking about. You're reading things, not seeing them. You think you know, but you probably don't. If I showed you two dogs, just their temperaments, would you be able to tell me what "line" they are? Unlikely. I can't even do that. I can tell somewhat based on looks, but even that can get sketchy.
So when you start spewing "internet research" to people that actually work their dogs and have been doing this for years...its almost comical. And some people will get angry. If this breeder has spent time emailing you back and forth, thinking you're a customer, now you tell them you read some article and changed your mind...I can see how they could get mad.
That is probably a major factor, i really was clueless when it came to the generalization of the lines. A lot of that is still seen on the forums some people will make a generalization about DDr or Czech OR west german and you will have people contradict that based of their own experience. I'm a first time GSD buyer so I can admit i have had no experience with any of the lines prior to my search and what i have read.