The women I've known who wear the hijab (some of them are Arabs, some are Asian, some are African and some are blonde-haired-blue-eyed Americans) have different ideas of what constitutes hijab, and different reasons for wearing it.  All I've been able to surmise is that, for most of them, this decision is personal.

A number of non-veiled women I know can envision a time when they will decide to wear it; when I asked one friend and colleague why she doesn't wear it, she said she just didn't feel God required it of her at this point in her life, but she indicated that it's possible that feeling will change someday.

I also have a very good friend who started wearing the hijab in the '90s, when there was a big veiling trend in Egypt, and then she took it off a few years later.