Hair can be a subject when in examples below it

1. Gets large in the frame.
2. Displays a photographic them: "Diagonal" (girl scout) or "Unusual" (spikey scout).
3. Covers the face, which makes the person abstract and archetypical.  Example, the whittling scout below.

See also how expert photgraphers portray Hair, Hair 2007.
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Boy Scout Troop 174, Yorktown, NY.  Scouting Photography Pages at  http://troop174.info