Anytime you ask people about some of the happy moments in their lives, they have to really think about them. However, if you ask them about some unpleasant or sad moments, they will tell you hundreds of instances.
My mother after reaching 94 years of age started hallucinating a lot. Most of the episodes are of fear. Fear of losing her jewelry; somebody robbing her; how the small girl child is crying (could be her childhood memories); etc., etc. When she is lucid, I have asked her why she is afraid when she is living a good life with everything taken care of and also asked her why she cannot think of some pleasant thoughts. She says there are hardly any! It is as if all the pleasant thoughts and memories have simply vanished or have gone so deep in her brain that she cannot access it.
What is the basis of such a thinking process? Science of neurobiology may throw some light on it.
There are ample indications that bad or unpleasant thoughts occupy major portion of our thinking though one is not sure of the exact numbers. There is socio-biological reasoning for it. In order to survive and continue living humans developed strategies to get resources and ward off enemies, predators, etc. This helped in developing the controlling instinct which allowed us to control our environment, hoard resources and other things including wealth. All these came with the baggage of insecurities and bad thoughts of harming others and in turn being harmed by them. These thoughts, insecurities and traumatic events produce very powerful memories in our brain.
As we age the percentage of anxiety driven thoughts increase since the life-time of anxieties overshadow the happy ones. That could explain the fear driven hallucinations of my mother. Besides with age the will power also reduces which in turn reduces the chances of accessing pleasant memories.
It has also been observed that once the negative or anxiety driven thoughts kick in more and more negative and angry thoughts start emerging from the brain. It is as if the brain goes in spiral about negative thoughts. This also starts in some people the episodes of depression.
When the ego goes in overdrive either because of adrenalin flowing or other inputs then large number of random neurons fire and neural pathways associated with existing memories light up. And since the anxiety driven memories are stronger and much more than the pleasant ones, this could be the reason why we get one bad thought after another and leads to an agitated mind. The stronger the memory is the least resistance it offers for neural circuits.
The calmness of mind according to Patanjali comes when we focus on a single thought for a long time. This process helps the brain to loosen other mental knots and allows the processing power of released neurons to focus on a single thought and help in reducing traumatic memories. This process gives tremendous happiness to the person. This is also the genesis of Vivek or wisdom which allows the brain to make decision after looking at all eventualities and helps in minimizing the painful thoughts. Naturally this yogic process is achieved by tremendous discipline and years of practice.
A simpler mechanism to reduce the production of negative thoughts is to think of happy thoughts. Even Patanjali talks about it when he says “To be free from thoughts that distract one from yoga, thoughts of an opposite kind must be cultivated”. This is a far better strategy then suppression of thoughts since suppressing them results in increasing the number of psychological knots. The suppression does not allow the thoughts to go away but only helps them to form powerful memories.
Happy thoughts unfortunately do not produce powerful memories but with continuous effort of cultivating them they can replace anxiety driven thoughts. That is the basis of all yoga which is to produce happiness.