Keep in mind that aggressive and fearful behavior, such as growling, snapping, biting, stiffening, and cowering are not normal behaviors in puppies.
A healthy and psychologically sound puppy should be naïve and at least somewhat eager to interact with people and animals.
Mild hesitation in approaching unfamiliar environments, noise, people, or objects is appropriate, but profound fear is indicative of a serious problem.
A puppy who takes more than 1-2 seconds to recover from mild hesitation, makes repeated escape attempts to remove himself from a situation or interaction, or who refuses delicious food or treats in certain situations is showing signs of fear.
Avoidance or hiding from people, animals, or objects: Avoidance is an indication of fear which may progress to aggression during adolescence and social maturity.
Alarm barking, lunging, putting hackles up in response to people or animals: This fearful response is not normal in a puppy who has no history of a negative or frightening experience in such situations.
Excessive mouthing specifically during physical handling: Examples include a puppy who mouths hard during toenail trims, the removal of a toy or object, veterinary exams, hugging, and lifting.
Reluctance to sit or down during training: Often this reluctance is due to PAIN.
Confinement problems: Excessive vocalization in crate or when home alone, an inability to settle in crate, and refusal to eat food when confined or in the absence of the owner may all be indications of early separation or confinement anxiety.
Repeated urination or bowel movements in appropriately-sized crate: If crate training is done appropriately and your puppy is crated for less time than it can be reasonable expected to hold it (1 hour per month of age + 1), repeated elimination should not occur.