Why is my dog aggressive toward me?

It depends on what the situation is. Are they aggressive around a certain object, food, treats, bone? If yes, your dog is likely resource guarding. If it is around a certain person, area, or crate/bed, they are likely being territorial.

Keep an eye on your dog's body language to identify what is triggering their aggression. It can often be something very subtle that could trigger your dog into some aggression so be extra mindful of how you interact with your dog. For example, a little move, like bending forward while asking your dog to sit could be the trigger the dog has learned for that means sit, instead of the command sit.

I would recommend a vet visit for your dog though, tell your vet about the aggression in detail and have them check your dog out. A dog that is in pain can act out in an aggressive manor and it is better to see if there is something medically underlining with your dog before moving forward with training. A good training would also recommend a vet visit prior to make sure your dog is okay. After that I would recommend training :)