Training a dog isn't hard it just takes time and that is where people fail the most. Also, subscribing to incorrect theories also is where people make a mistake. Mainly pack theory and dominance theory (alpha).
I have been professionally training dogs for close to 10 years now. I would be considered a "cross over trainer" since I originally subscribed to the dominance and pack theories. As soon as I found positive reinforcement training things clicked (no pun) for me. I started clicker training and saw such an improvement in my dogs behavior.
Once you or anyone understands how dogs learn, training is easy. There are a lot of old theories and methods that are wrong and still used today which is the problem. Don't get me wrong, punishment/scolding works, but positive reinforcement works better.
Lastly any dog can be trained but some breeds take to training better. "Working" (Austrailian Shepherds, cattle dogs, border collies, etc) dogs tend to pick up on things very quickly. But if these dogs don't have something to do like training, herding, flyball, agility they can be real trouble makers.
Why you and your dog are not a pack.
The scientist who coined the term "alpha" several decades ago discusses why recent studies show they are inaccurate terms for describing wolf behavior--much less dog behavior!
Dumbed down by dominance