"Don't worry, be happy."
This well-meaning piece of advice was the chorus of a popular 80s song by Bobby McFerrin. It also probably represents the general philosophical attitude of western society in terms of how to maximize our quality of life.
There is a wonderful intuitiveness to such advice. Worry and other negative thoughts cause negative emotions, which are not pleasant. Conversely, happiness and thinking positively feel good. Easy choice, right?
"Think positively" and "the power of positive thinking" have become trademark slogans in our society. From self-help magazine articles to motivational speakers to friends and family, it has become folklore wisdom -- fill your mind with positivity and you shall reap the mental health benefits.
In fact, negative thoughts and negative emotions tend to be seen as akin to germs and viruses -- things to be avoided and fought. For many people, positive thinking is like medicine for one's mental health (a cheaper, side effect free version of antidepressants).
The problem is that while positive thinking can yield some mental health benefits, an excessive and rigid search for positivity can bring about the opposite effect.
Sticking with the analogy of germs, negative thoughts and emotions (including stress) can be a good thing in moderation -- like germs or viruses that stimulate the functioning of the immune system. There are a number of benefits of negativity, and at least some negativity in one's life from time to time is probably desirable.
First, negative thinking can be adaptive. When problems arise, worry can be a constructive thing if it leads to problem-solving. Anxiety is useful when we are threatened and are in need of safety. (For example, you should feel some anxiety when driving in poor weather.) Sadness is a normal emotion in the context of loss. In fact, it is believed that the symptoms of depression evolved to facilitate the need for rest, protection and self-soothing.
Second, negative life experiences (including negative thoughts and emotions) often play a significant role in maturation and character development. Guilt and shame not only allow us to recognize and correct mistakes we've made, but also to become a better person. Repeat experiences with frustration help build tolerance and may ultimately assist in the development of patience.
Third, the negative in life makes possible the enjoyment of positive things. Our perceptions are often shaped by contrasts. If you move your hand from ice-cold water to lukewarm water, lukewarm feels hot. A positive life experience will be more intense and meaningful if it occurs when stress and other problems have been in the background. When life is perfect, it is more difficult to enjoy the good things. Generally speaking, the threshold for receiving pleasure and enjoyment from experiences is more difficult to reach when pleasure and positivity are the norm.
Just as an immune system can become stronger with exposure to germs, people can become more capable of coping and enjoying life when they have had some exposure to negative life events (like a vaccine).
If negativity is a germ, positive thinking is Purell -- certainly useful and needed, but some wonder whether its overuse weaken the user? Research shows that when people experience very few negative life events, they have less life satisfaction and more distress than those who have had to cope with moderate amounts of stress.
Some people avoid negativity like they are trying to avoid a dangerous microorganism, and conversely strive to be surrounded by "positive energy" -- a common pop culture term that I admittedly do not fully understand, but it sounds like a wonderful shield against the trials of life. (Oh, if only Schopenhauer had had more of this energy, how differently his life would have been!)
Now, allow me to be clear. I am not arguing against positive thoughts. They certainly have their place in mental health and can be very helpful and important.
However, it is probably best to use adaptive thinking instead of rigid, positive thinking. Adaptive thinking is defined by one's ability to use whichever thoughts are best suited to a given situation. Sometimes we need to worry and feel anxious. Focusing on the positive while failing all of your university courses is not an adaptive strategy.
Conversely, trying to obsessively identify your faults (ie., self-criticism) and exaggerating the meaning of mistakes serves no useful function either.
Ideally, the goal is to utilize the most useful thought, emotion or behaviour -- regardless of whether they are "positive" or "negative." Psychologists have referred to this approach as psychological flexibility, and it is a significant predictor of psychological well-being.
Being flexible with positive and negative psychological states of mind can be difficult to achieve, but is more likely to yield significant benefits in the long run.
Finally, it should also be noted that if it were easy to simply stop one's worries and just "be happy," I'd be out of a job.
For more by Roger Covin, Ph.D., click here.
For more on mindfulness, click here.
 Allen & Badcock (2003). The social risk hypothesis of depressed mood: Evolutionary, psychosocial, and neurobiological perspectives. Psychological Bulletin, 129, 887-913.
 Seery, Holman, & Silver (2010). Whatever does not kill us: Cumulative lifetime adversity, vulnerability, and resilience. Journal of Personality and Social Psychology, 99, 1025-1041.
 Kashdan & Rottenberg (2010). Psychological flexibility as a fundamental aspect of health. Clinical Psychology Review, 30, 467-480.