This is dominance aggression. He is maturing into an adult and testing you. If he can control you by growling he will. If he gets his way, he will continue the behavior. Continue obedience training. Even if he knows his commands he needs the daily training to keep seeing you as the boss and dogs do not growl at the boss. Look at a group of dogs that live together. One dog gets to do anything they want and take things away from all the others and none of the other dogs growl or complain because that dog is the boss. You want the dog to see you as the boss. Now he doesn't or he would not growl at you.
Let me give you a site on obedience training. The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
Now there might also be another reason. Poodles, chihuahua and dachshunds suffer from disc issues so being picked up often causes them discomfort which can lead to them growling from pain. Many people pick up dogs like they do children under the front legs. This puts undue pressure on the discs in the back a the weight of the rear of the dog pulls down on the discs. The proper way of picking up a dog is to place one hand on the chest of the dog between the front legs and scoop the rear legs up in the other hand while lifting straight up. This keeps the back straight and the discs won't hurt.
With the obedience training you should be able to get your dog to do what you want without picking up your dog. Limit picking them up to when it is absolutely necessary. He'll learn quickly that you have changed and you will not see that behavior if pain in the discs was the cause.
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