Animal behavior is complicated. Unfortunately, there is a lot of misinformation out there. A lot of lay trainers use negative reinforcement or positive punishment techniques. There are some very famous TV dog trainers who use techniques that can lead to dangerous situations for the owner and the pet. Please follow the links below to decide how to approach training your pet or addressing behavior problems.
The American Veterinary Society for Animal Behavior has some good information at the link below:
When choosing a dog trainer, please make sure to find one using positive reinforcement and that is certified. Below is a link to finding out if your trainer is certified:
Here is a link to a video that helps you decided if your trainer is using the correct positive training techniques:
Finally, a good resource of information is Dr Sophia Yin. Her book is available for purchase, she has a blog and archive of information, but she also has some videos on youtube that you can search for.