If you don't lead, they will.

Your dog needs a leader. If you don't do it, they will. In most cases, they'll do it badly. Over the thousands of years since we first started domesticating the grey wolf into all the modern variations of dogs they've only become more and more a part of our lives, homes, and families.
 They now exist in a world that has rules and circumstances that are far removed from what their instincts and impulses tell them to do.
Your dog may not understand why you ask them to sit when you reach a curb, come to you when called, or wait patiently before diving in to tasty smelling meal, but we know that we do these things to keep them safe, healthy, and happy.
 It should also never be discounted how much our dogs are still pack animals who thrive when there is a clear leader in their lives. If their humans don't provide consistent structure and leadership dogs will seek to fill that void themselves. This tends to lead to all sorts of behavioral issues, from anxiety to aggression and all points in between. One of the most important pieces of advice that I always pass on to new clients is that every moment we spend with our dogs is training, whether we intend it to be or not. How you respond when your dog jumps up on you, scavenges the counter, barks at the mailman, or loses their mind about another dog walking by is a little training opportunity dropped into a random part of the day. By setting rules and expectations with clear rewards and consequences we enable our dogs to make decisions that both make sense to their instincts, and work in our world.