The latter result is due to the fact that rumination builds complex interconnections in our brain that (mainly due to the activity of dopamine neurons that are active when we contemplate problems) increase the efficiency of thinking, and as it turns out, enables us to resist degenerative diseases of the nervous system, namely dementia or Alzheimer's. This was confirmed recently by Israeili researchers who discovered that a consistent lack of rumination associated with much higher incidence of dementia in later life. Thus, if you don't think about your problems you stand a 40% increased likelihood of dementia later in life. The irony is that those who worry about this don't need to worry, and those who don't worry about this do. A no win or should I say no worry situation is there ever was one.
All of this goes against the grain (or should I say brain) of the don't worry, be happy school of thought (e.g. mindfulness meditation, positive psychology) that sees the future of humanity as a tranquil field of flower sniffing clones. Supposedly, if you're happy just non pensively looking at the world, you'll be even more happy as a human vegetable. It makes sense if your aspirations are to become not a couch potato, but just a potato.