Why did the Greasers in "The Outsiders" wear greasy hair?

It isn't so much that they wear greasy hair, but that they put grease in their hair to slick it back.  This was a status symbol, as was the long hair that many of them grew.  It was for them what Hollister clothing is for many teenagers today - a way to express themselves and also identify themselves.  It was also a way to seem more "tuff", as it is put in the book.  The slicked back look was in contrast to the more preppy look the Socs were wearing, and it helped the Greasers feel as if they were rebelling against the society that so often put them down.