Like most things human, I think the effects of isolation and/or loneliness must fall on a continuum. I do think that there are those humans who are the sentinels of the bunch - the natural loners. Evolutionarily speaking, those would serve as the lookouts for the group. The scouts. Or those who, for whatever reason, do not get the payoff from social interaction.

I do not think this is aberrant or even harmful to those individuals. Not all seek or desire the tribe so to speak.

I do not agree with the author's contention that there is an addiction-like quality to negative thinking. In fact, just the opposite I believe to be true. The relief from that stressor is far far far more powerful than the experience of stress. Also, the release of endorphins only mitigates the effects of stress - or there would be fewer negative effects of stress. With time, stressors become centralized and the endorphin response blunted. It just doesn't ring true to me.

I think humans are hard-wired to feel better, not worse. We seek relief because we can't provide it endogenously.