Regression to the Mean

'Regression to the mean.'
Supposedly, it means that things don't stay bad or good, that they always go back to the middle.
Someone said it in a really lame werewolf TV show, which I loved so much, not so long ago.
Until recently, I thought it's a really clever saying. But lately it seems to me that it doesn't always have to work that way.
What if we actually get to live our dream and not come back out of it? Maybe our lives aren't rollercosters...
Or maybe, sometimes, some nightmares never end. Maybe they last forever.