The stigma and fear of needles is well documented - there is even a listing in DSM IV that refers to needle phobia. My sister used to faint every time she had an injection at the doctors - even as a teenager.
Also - media representation of needles and syringes is overwhelmingly negative. They are either shown in a medical context - with white coats and face masks - or associated with deviant behaviour - concealed faces, other indicators of social deviance. Also, any media photo of a needle/syringe is inevitably of a large, dirty syringe with a crusty monster horse needle attached - this adds to the stigma.
Interestingly, this stigma attached to injecting could actually encourage problematic use (see Zinberg "Drug, Set and Setting"). If you inject, you might have to hide it from your friends, might start hanging out more with people because they are injectors, rather than actual friends, might be exposed to more dependant users etc etc.
Personally, I don't inject - but I know plenty of people who do or have. I just don't let it affect my relationship with them. They can (and do) do it in front of me and I won't care. I may or may not choose to inject in the future - but if I do it will be safely.
"Drawing lines" is interesting (see satricion above) - funnily enough even people who inject draw lines: "I'm not a junky - I only inject speed", "I'm not a junky - I have a home/girlfriend/job". And of course it is very common for non-injecting illicit drug users to distance themselves from injecting. Basically, when you get picked on, it is common human behaviour to find someone else to pick on in return to make yourself feel better.