Performance Art and Dadaisim

Personally I do not understand Performance art so well. Performance Art though has been around for quite some time, involving the Dadas, Nouveau Realists, Gutai, Fluxus, and even the Bauhaus in Germany. Performance Art is live and has no rules or guidelines, a way for artists to break away from society even more and express themselves. But what is it they are exactly expressing and trying to let us know is hard for me to understand through the Performance Art. What was Alan Kaprow trying to get through to his audience with his performance with the mirrors? Performance Art is called art because the artist says it is art. For that matter anyone can do anything and call it art, this perhaps is what the pioneers of Performance Art were trying to do. For example, the Dada movement began at the onset of WWI as a negative response to society and the conventional ideas of art. Dadaists went against the contemporary academic and cultural values of art. They wanted to bring art to the next level, to go against the traditional values and bring their art out onto the streets and have participation of an audiance. Dadaisim was not just confined to the visual and literary arts, it involved sound and music. A Dadaist know as Kurt Shwitters developed a Performance Art known as sound poems. Musicians collaborated and made Dada music and the members of the Dada movement performed their works at Dada gatherings. Many of the performance art that I have looked up and watched, and have know about I still don't quite get. Like Yoko Ono sitting on stage and random people come up and can cut of parts of her shirt as she just sits there and stares into space. How is this art? What is it we are supposed to get? Why is this called art? Perhaps with further knowledge on this subject I can fully understand the age old excitement and meaning of Performance Art.