I think more and more that thinking too much is one of the most detrimental things to health. Thinking as a whole seems to be stressful to the organism.
I also am starting to think this is extremely true. I think mental chatter is inversely related to thyroid function. Estrogen and PUFA stimulate the part of your brain responsible for mental chatter. Conversely, keeping an empty mind nullifies the effects of estrogen and PUFA. This is why mental chatter gets worse with age and kids don't really live in their heads. Of course, the fact that their parents are caring for them and they have no stress/responsibility plays a role in this, but I think there is a more physical/biological reason as well.