We all want our dogs to be civil, loving, easy to handle members of our family. To act like gentlemen and ladies when they are in public, and to be seen as "well trained" by those who pass us by. If we are using our "big, reasoning human brains," (thank you Zac George for that turn of phrase) we have made the decision to enroll our new dog in some kind of class, or hired a private instructor to come to the house. We start to make progress, but it feels slow. It doesn't feel "all encompassing," or the Teacher we hired doesn't show us the "fun stuff." So, being the curious species we are, we start looking at what else is out there; what else we can get involved in. We are doing Simple Obedience, what about Agility, I heard that's fun. Or how about Nosework? Tricks? Rally? Protection? Conformation?
Wanting to do all of these things is Great. I encourage you to do so, but please, for your dog's sake, Not All At Once.
The thing about taking multiple classes are the Instructors. Trainers are a lot like Hair Stylists: We may all be cutting hair, but we all do things just a little bit differently. This becomes VERY confusing for your dog. This confusion can lead to what may look like a lack of cognition, or stubborness. Outright refusal to do what you ask may not be a stubborn dog at all, but a confused animal who can't speak the language.
Take something simple, like teaching a "Sit." One Teacher may ask you to hold a piece of food in front of your dog's nose, and slowly use it as a lure, to cause him to raise his nose and follow it back over his head a bit. This causes his butt to go down in to a Sit position, and then you give him the treat.
Another might suggest you hold a treat in your closed fist, a bit above the dog's head out of reach, with your arm extended straight in front of you. Say "Sit," then patiently wait for the dog to stop jumping, rearing like a pony or otherwise trying to investigate your hand. As he looks up at your hand, he may sit. When this happens, you say "Good" and give him the reward.
Here comes somebody who shows you to position the collar just behind his ears, and apply upwards pressure (slowly choking him) to his leash, while you press down on his butt. Thus creating leverage so he begins to go in to the sit position. Release of pressure happens as he begins to drop his haunches, reapply this pressure should he begin to stand.
Still another comes 'round and tells you to simply wait 'til the dog sits all by himself at some point. When he finally does, you are to repeat "Good Sit" over and over again while you reward him. Then ignore him again and repeat this process a few times. Once he is offering this behavior to get the reward, and you think he is starting to understand that butt on the ground equals food, you can then experiment by asking him to "Sit" when he is standing, and see if he puts it together, thereby making it a cue, and not just free food for a randomly offered behavior.
Oh and then there's using a clicker to get the behavior on cue.....
You see? So many different ways are out there to teach even the simplest of things, that there are bound to be enough inconsistencies to create confusion in your dog. I am not saying don't take these classes. I want you to. Just take them one at a time, unless they are all from the same Instructor. That way, if you do encounter a Teacher who is using a different approach, your dog will have time to adapt to this change, and succeed. They are extremely capable of learning different styles, and cues, but it must come at a speed they can handle.
Think of it as though you were trying to learn two different languages at the same time (and yes, I know, some of us out there are AMAZING humans who actually can do this...I speak to the majority here). On Mondays, you attend Russian class, and Wednesdays, you go to a Mandarin hour. You may recognize some of the gestures and tones that those two instructors have, but they are so very different, it may be near impossible to be successful. Keeping up with the rest of the class in both languages may be more of a challenge, and you may find yourself falling behind and feeling frustrated.