I've googles this many a time, and one theory is that it's caused by mites, which we all apparently have living on us, only some people have too many (or something).
It tends to run in families, and often goes away with age.
Afair, any treatment has to be ongoing, which is why I've given up on the daily scrubbing (can't be arsed) - just use Flexitol when they're on show
I'm sure I'll now be trying all the above cures, though..