i think it's cos they're dead inside - ie. both lost a loved one.
i have a snippet of info that i gleaned off the richard & judy show when they were interviewing the korean guy, the fat guy and mr. echo.
don't read on if you don't want to know
but if you slow down the black smoke, you see images of that person's life, so it's like their past. are they running away from their past, or have they faced up to it?
my own musings: mr. echo had reconciled himself to his past and exonerated himself, now he sees himself as a priest and so he could face the black cloud. charlie hasn't sorted out his past / drug addiction, so was scared of the black cloud.