The "hippie" subculture in the United States came about during the 1960s, as a youth movement and counterculture movement. Counterculture movements that leaned towards communal living, and the ideals promoted by more modern hippies, existed as far back as in the 1800s in Europe (specifically Germany) as a push-back against urbanization and a return to the natural, spiritual life that their ancestors lived.

If you're referring specifically to the aesthetic, the cultural movement took inspiration from what they considered more natural colors and patterns, breaking from the suit-and-tie dress that was the norm of the day. That extends to the long hair, and unusual attire such as ripped jeans or the circular sunglasses. The influence from nature is evident both by the bright colors, and floral designs that were prominent on much of the clothing that was worn by hippies during this time period.

Additionally, the psychedelia movement, that came up hand-in-hand with the hippie subculture, was influenced by the usage of psychedelics such as mushrooms and LSD, which used distorted visuals and bright colors to try and evoke emotion or feelings through the art. Things like tie-dye (a technique that dates back as far as 500-800AD) gained popularity during the hippie movement, because of the ability to use the technique to create vivid and bright patterns.

One of the final influences was the war in Vietnam. It was seen as unnecessary by many people, and the ideals of peace, love, and equality was very prominent within the hippie subculture, which in turn influenced their clothing styling of bright colors, floral patterns, and the love beads. This is a very basic outline of where the modern hippie aesthetic came from, and how social events at the time further affected this subculture. I urge you to research it more if you have the chance, as I only touched on the visual aesthetic that was popularized at the time, and didn't get into much other than that. Hope this answered your question!

Pendergast, Sara. (2004) Fashion, Costume, and Culture. Volume 5. Modern World Part II: 1946-2003. Thomson Gale. Tompkins, Vincent, ed. (2001b), "Hippies", American Decades, 7: 1960–1969, Detroit: Thomson Gale.