I'm just going to leave these quotes from behavioral experts here. Experts with the science to back them up.
"To use shock as an effective dog training method you will need: a thorough understanding of canine behavior, a thorough understanding of learning theory, and impeccable timing. And if you have those three things, you don’t need a shock collar." - Dr. Ian Dunbar
"Shock collars induce pain and suffering and are often overused. The pain from the shocks is on its own, a serious welfare issue. But, in order for a dog to learn from these devices the shock has to be of an intensity to invoke fear. As the dog starts to habituate to lower levels of shock, the handler will usually increase the shock to get the dog to respond. The dog doesn't know from where the shock is emanating, so its stress hormones and heart rate will escalate creating significant physiological as well as psychological stress. So they are not only experiencing intense pain, but intense fear. As the shock is remote and the dogs have a problem identifying which of their actions are generating the shock, they will often then generalize their fear to a broad range of other neutral things that they believe are associated with pain. With electric fences, dogs may become fearful of going outside instead of just near the boundary of the property. More importantly, the shocks will motivate the dog to escape the stimulus that is causing the pain resulting in defensive aggression. There is evidence from invisible fence studies that dogs redirect this aggression towards the nearest human, causing serious injury (Polsky, 2000). Alternatively, if they believe there is no escape for the pain, a type of learned helplessness or psychopathology resulting in self-mutilation or other destructive behaviours could result." - Dr. Jill Taggart, Animal Behaviorist
"Electric shock is a uniquely dangerous form of punishment because it's invisible to the punisher. People have difficulty gauging the level of discomfort or suffering the animal is experiencing. The pioneering research conducted by Yale professor Dr. Stanley Milgram 50 years ago demonstrated how willing most people are to inflict dangerous (and even potentially lethal) levels of shock on another person if convinced this will help him learn. These data are directly relevant to the often disastrous choices people make when trying to "educate" their dogs.
The good news is that shock collars are archaic and unnecessary. Suppressing "bad" behaviors through the use of shock and other physical punishment may seem like a quick fix but is never a long-term solution. As an alternative, skillful training builds calm and cooperative behaviors in dogs through the structured use of positive reinforcement." - Kathy Sdao, M.A. applied animal behaviorist
"As a specialist in veterinary behavioral medicine I have been advocating for banning the use of shock collars of any kind for years. There are now ample published data in the peer-reviewed literature that show that shock harms dogs and adversely affects their behavior and welfare. Many people who resort to shock are afraid that without it their pet will die because of their behaviors. The companies who sell shock collars prey on these fears. Most people do not realize that the use of shock interferes with and suppresses normal canine behaviors, in general, not just behaviors that people find problematic. In fact, shock may render the behaviors of concern worse. In my patient population, dogs whom clients have shocked are over-represented in those euthanized because of the adverse effects shock has had on their behaviors. Dogs can recover from shock with appropriate care, and anyone considering shock should first seek the help of a qualified specialist in veterinary behavioral medicine. Specialists should be the source for competent, data-based information for any behavioral issues about which clients are concerned. I have studied the information provided by the companies manufacturing and selling most of the world's shock collars and it is my opinion that, without doubt, the information provided about behavior is incorrect and/or inadequate to address the behavioral concerns of dogs and may lead to abuse. The time to advocate for safe, effective, humane behavioral care for all animals has come, and shock has no role in such care". - Karen L. Overall, MA, VMD, PhD, Diplomate American College of Veterinary Behaviorists, ABS Certified Applied Animal Behaviorist