Who's the boss? Well, you are, or you should be. But before you start "alpha rolling" your dog or "showing him whose boss," please read on.
Contrary to what you'll hear from a lot of trainers, not only is it unnecessary to "dominate" your dog for him to know you're his leader, but it can be downright dangerous. Dogs perceive scruff shakes, so-called "alpha rolls," and other physical discipline as attacks, and dogs naturally respond to attacks by defending themselves. A very submissive dog may cower instead of attacking (but why are you disciplining a submissive dog in the first place?).
However, a pushier, more confident dog may well ..fight back, which can lead to a dangerous escalation in punishment: you punish the dog, he growls; you punish harder, he bites; you punish harder, he really goes for you. The end result may well be a dog that doesn't trust you or a constant battle of wills. In my experience, most aggression problems in dogs are owner-caused, usually by inappropriate punishment. And it's simply not necessary. Dogs don't respect leaders who physically dominate them, but rather those who provide access to what dogs want. For most dogs, this means food, toys, or the opportunity to go outside, to play, and to go for car rides or walks. You control the dog's access to all these things, so you hold the leadership position. How you use that position is up to you.
When you are taking your dog outside, ask him to sit at the door. With the dog on lead, open the door slightly. If he gets up, just say, "Excuse me" in a quiet voice as you step in front of your dog and move him away from it. Pretend the dog is a soccer ball and you are the goalie, don't let the dog through the door and move him back away from it. Close the door if you have too. Wait until he looks up at you wondering what's up! Now try again. When he has backed off from the door open, say, "o.k." and head out the door first with him following you outside. Most dogs figure out in one session that the door opening doesn't give them permission to just charge outside. You've not only taught the dog to wait at the doorway - you've also proven yourself worth listening too and a good leader! Any time you train your dog using food and other types of positive reinforcement, you are proving your leadership.
Reduce the dog's meals so he's hungry (not starving) when you train. Use lots of food in training, and make sure the dog does something for every morsel. No more free food! The dog will welcome this change. Dogs are hunters, not grazers, and hunters must work hard for each and every meal; getting free food is simply not natural for them. Doing something for their food will give your dog an increased interest in food, an increased desire to perform behaviors for you, and an increased respect for you as the provider of what it wants and needs. Just asking for a sit before putting down the food bowl at each meal can prove your leadership.
And what about all those "rules" of training, like "don't let the dog sleep on your bed, "never let the dog win at tug-of-war," and "never let the dog train you"? Well, like most "rules" there's a little truth to all of them, but only a little. At our house, all four dogs sleep on the couch or chair. But not until they are housetrained, are capable of sleeping through the night, are no longer chewing everything in sight, and have learned, through short training sessions, to get on and off when asked. Its not there couches its mine. This usually means that puppies in our house sleep in their crates next to the bed until the age of 8-month - 1-1/2 yrs of age. Likewise, we play tug-of-war with all of the dogs its one of their favorite games. But we didn't do it until thye had been trained, with food, to drop toys on cue. Sometimes we let them win and sometimes we ask them to drop the toy - either way, they have fun. When we are done the toy gets picked up and put away till the next time.
Finally, I can't imagine anything better than letting a dog train you! My clicker-trained dogs have actually learned something about training by being trained, and I'm happy to let them train me sometimes, too. Turnabout is fair play. Obviously, I pick and choose which of their "cues" I will respond to. When I don't respond, they stop trying to make me do that particular behavior and try something else. All of my dogs love to "make" Kevin (my husband) give them a biscuit first thing in the morning and in the evenings. And all the dogs can "make" me take them outside for agility practice by leaping around my feet and running to the door.
In the final analysis, it's much more productive to look at dog training through the metaphor of dancing rather than that of domination. When two people dance, one must lead and the other must follow, if both try to lead, they'll fall on their faces! But the best dancers show complete harmony and teamwork, not dominance and submission. And this is what I strive to achieve with my dogs - the harmony and joy of teamwork.