Again....not always true!! Someone can have 5 children..all raised the same way...and one turns out bad, or turns to drugs, jail, etc. Does that one child maybe have a personality (genetics), that can make a child that was raised right...go the wrong way???
You are right, dogs are not born aggressive...but some have it in their genetics to be stronger in aggression, or be more fearful, or more dominant, etc. I fostered many Rottweilers coming out of shelters...very little background. Some came out of the shelters like black/tan Goldens...loved everyone, happy friendly,etc. That is not the breed standard. My Jemma came from Yugoslavian lines on her dams side...dogs titled in IPG, sire's side was more easy going. She was raised right...socialized, into classes from 11 weeks of age for many years, etc. She was still a suspicious, and protective dog. She was great with our own family, and a few close friends...but that was about it. This is where genetics come out. You can't train out, or love out genetics.