Your dog perceives your behaviour as weak and your dog will go into protection mode. Behaviour problems in dogs, which are not solved, can lead to aggressive dog behaviour.
Dog behaviour problems like frustration, Insecurity, anxiety, fear and a lack of socialization can lead to aggressive behaviour if not solved.
Frustration: Your dog experience frustration when you do not meet your dog’s needs, especially when your dog is not going for daily walks and getting into a daily exercise schedule. This frustration will worsen if your dog does not have any boundaries or discipline.
Insecurity, anxiety and fear: Dogs that are insecure, anxious and fearful will not naturally fight but they will rather run or hide. Unless danger is removed, they can lash out aggressively. If your dog is in a constant anxious state, it can react aggressively to everything.
Lack of socialization: When you prevent your dog to socialize with other dogs, your dog will become antisocial, especially when you keep on picking up your small dog at the first sight of any danger. This confuses your dog and they associate being picked up with affection and that THAT is what you want them to do (barking and lashing out towards other dogs or things or people).
Dog aggression is not natural. You teach it to your dog. Become very much aware of what you are doing to make your dog antisocial.