From Sonja Lyubomirsky’s excellent book The How of Happiness,

Many of us believe that when we feel down, we should try to focus inwardly and evaluate our feelings and our situation in order to attain self-insight and find solutions that might ultimately resolve our problems and relieve unhappiness. Susan Nolen-Hoeksema, I, and others have compiled a great deal of evidence challenging this assumption. Numerous studies over the past two decades have shown that to the contrary, overthinking ushers in a host of adverse consequences: It sustains or worsens sadness, fosters negatively biased thinking, impairs a person’s ability to solve problems, saps motivation, and interferes with concentration and initiative. Moreover, although people have a strong sense that they are gaining insight into themselves and their problems during their ruminations, this is rarely the case. What they do gain is a distorted, pessimistic perspective on their lives.