If you're worried about your Rottweiler being or becoming aggressive based on its pedigree alone, then you should know that dog breeds commonly associated with aggressiveness - Pit Bulls, Dobermans, and German Shepherds - are not inherently so. Many canine behavioral researchers and experts agree that an aggressive dog became that way solely through negative life experiences, lack of training, and/or isolation. It's safe, in fact helpful, for you to set these fears aside. The key to raising and owning a non-aggressive dog is to socialize them as puppies and give them plenty of love, attention, and training as adults. The more boundaries and rules you can set for your Rottweiler, the less likely it will be that he exhibits aggressive behavior.
In addition to basic socialization or desensitization methods, you will also need what is called 'counter-conditioning', which reroutes how a dog thinks about and reacts to different things in the world. These training methods can take anywhere from just a few months to a full year, so it may take some time to see results. However, with enough time and patience, and by starting as early as possible, you can prevent your Rottweiler from developing aggressive habits.