1. Parrots are very intelligent animals, nature developed their brains over millions of years to learn how to think and solve problems in order to survive and look for food. Since in captivity we take away these basic needs from them (and often even most of their physical exercise) then they simply dont know what to do with all the brain power, get bored and frustrated because their best pal doesn't understand their language and in human eyes "look for trouble". I believe we can prevent many behavioral issues by keeping your parrot "thinking" (of course together with proper exercise and foraging, but that is covered in different articles). What I mean by Training doesnt necessarily have to be extra advanced tricks so you become discouraged and it will not be fun for you. Spend at least 10 - 15 minutes a day training simple fun tricks, use your imagination and I guarantee you that you will have much better relationship with your bird than if you bought him lets say the biggest stainless steel cage or the most expensive toy. Even tho these are very nice and good, our animals dont really care for them as much as their owners, what they really want is to live in a flock where they can understand and be understood.
2. Every animal species have a level of communication, some more and some less. Dolphins, Elephants, Cuttlefish (here is a link to a Cuttlefish communication video, very neat) and Parrots are one of the most advanced from them. Some research indicates that Cuttlefish have more complex language than humans for example. I think most of us have the ability to only touch the surface when it comes to communicating with our animals, really and the almost only way to develop such language is what we humans call "Training". Here we at least are able to show our parrot what it is that we want him to do and if we like what he did and so on. I believe it is only Fair that we allow them to learn our simple "language" by training with them and really it is somehow unfair not to do it. After all it is us who chose to keep such smart and amazing "pet" and I hope ALL of you did this mostly for their personalities and abilities and not only for their beautiful feathers.
Example of Communication between Black Palm Cockatoos