The best way to lock in our dogs' behaviors is to put them on a "variable reinforcement schedule" (or vR+ in science lingo) - make our rewards for their behavior come at random times and in random amounts.
Metaphor: Vegas, baby! Gambling is fun (and oftentimes even addictive) precisely because you might win at any time, and the amounts can be different each time. And when you think about the behavior that is being locked in - "give us your money" - you can see just how powerfully vR+ works on us. Getting your dog to Sit on command should be a cinch, comparatively.
Often, a good indicator if you're pushing your dog too hard, or if they're feeling stressed by a situation, is if they start nipping your fingers hard when they take food from you.
Metaphor: Horror movies. Ever notice that when the protagonist is being chased by the killer and they're trying to get into their car or home, they always have trouble getting that key into the slot because their hands are shaking too much? When we're stressed, we lose control over our fine motor functions. Dogs are the same way. Normally, a dog has probably more control over its mouth and teeth than we have over our fingers, so if they're losing their normally gentle touch, they could probably use a break for a while.
It's a common issue for people who've recently rescued a new dog. For the first few weeks, the dog is perfect - quiet, respectful, well behaved. And then, after 2-4 weeks, it starts growling or nipping at people if they come near its toys, or it starts lunging at other dogs on walks, or starts chewing up the furniture. But it was so good! What happened?
Metaphor: Transfering to a new high school. Ever moved and had to enter a new school when you were a kid? At first, you don't know who's who or what the rules - explicit or unspoken - are. So you keep your head down and try not to draw too much attention. But after a bit, you start to figure out who the cool kids are, the different cliques, which teachers will let you slide and which are hard cases. And then you can start to let your personality show; crack some jokes in class, show off at recess, start making friends. Or,negatively, start picking on kids you see as easy marks or disrespecting the teachers.
The same goes for dogs. Those first few weeks in your home, your new dog is unsure about how you will react. Do you have rules you are stubborn and consistent about? Do you care if the dog is being good or do you just ignore it? And so, after a few weeks, your dog starts to feel more confident about its place in the home, and it's "true" personality will start to come out. The good news is that you can use those first few weeks as a blessing and reinforce the heck out of all that good behavior, so that when your dog starts feeling free to behave as it likes, it will want to keep acting like a good dog. Take that good behavior for granted at your peril!)
How is that dogs that start off so super-excited and manic about wanting to play with other dogs that they lunge and lunge against their leash can become extremely aggressive towards other dogs over time? How did "I love other dogs SO MUCH!!!" turn into "I'll kill you!"?
Metaphor: Lakers Championships. When the Lakers win a championship and the fans rush out of Staples Center here in L.A., they're all in an incredibly good mood. They're cheering and yelling, they're high fiving and chest-bumping, "Wooo! F#%& Yeah!" And then what do they do? They get into fist fights and set cars on fire. Because too much adrenaline and uncontrolled energy has to go somewhere. And whether it's people or dogs, getting too worked up without any self control is a recipe for violence. When your dog lets itself get so manic about wanting to play with other dogs and then is continually brought up short by the leash, it gets frustrated. And if it hasn't learned good impulse control to deal with that frustration, all that energy and frustration turns into negative emotions and aggression - just like a bunch of drunk Laker fans getting into fights with each other. (I stay off the streets after a championship lets out, if you can't tell).
What it all comes down to is that with just a little change in perspective, it's easy to see that humans and dogs are much more alike than we are different. And more than just helping us to empathize with our dogs, knowing this helps us understand ourselves better, too. So go chew on a bully stick and mull on that for a while.