Try to shoot people and you will understand that this is not so easy. Human beens have the most expressive nature of all creatures. When you shoot people is not about only of the moment or the feelings, but its about the relationship. The best photos ever made are full of relationship not only with the photographer, but because people o the photos are connect with something in their own world.
For example Winogrand Garry had remarkable style. "Winogrand was known for his portrayal of American life in the early 1960s, Many of his photographs depict the social issues of his time day and in the role of media in shaping attitudes. He roamed the streets of New York with his 35mm Leica camera rapidly taking photographs using a prefocused wide angle lens. His pictures frequently appeared as if they were driven by the energy of the events he was witnessing. While the style has been much imitated, Winogrand's eye, his visual style, and his wit, are unique."(Wikipedia).