I think research is a big factor, people will jump the gate and not do any research, your body only produces around 30mg of testosterone a week and some people run up to 1000% more then this on a first course.
Also things like alcohol lack of sleep and bad diet should be avoided at all costs as it will only restrict the gains they make.
Just because you inject doesn't mean you will get bigger, you have to work hard at all aspects.
This also applies when not on the juice.