Emotion.
This single word has multiple implications. It's different shades intermittently alter mental states, infused by what we call anger, distress, bliss, and like things. Some input triggers it. A good news triggers joy, a bad news infuses distress, depression, agony .
The news, good or bad, is the trigger. An input. The impact is realised in the form of some emotion, whether welcome or unwelcome, positive or negative, happiness or anxiety-pain-sorrow.
What goes unnoticed is the hidden fact that what, in fact ,   our  mental states are thus dictated by chemicals that the brain secretes ' by default '. That which occurs automatically, without individual volition, is called , 'by Default' . It is what the mind is trained into effecting. 
neurotransmitters and neurons in the brain are the agencies that produce these effects.
Man has evolved Antidepressants that work by adjusting the amount of neurotransmitters reabsorbed by neurons in the brain.

Human  brain is a complex network that processes vast quantities of information every second.
The limbic system that situates at the top of the brain stem is the pivotal organizer.
Part of the brain's information-processing network includes neurons, or cells that transmit signals throughout the brain. Neurons send signals through neurotransmitters, which are chemicals,  some of which are released  while the  others are received.
These chemicals essentially let the parts of the brain communicate with each other.

The three most commonly studied neurotransmitters are;
(1) dopamine,
(2) serotonin and
(3) norepinephrine.
Dopamine is related to experiences of pleasure and the reward-learning process. In other words, when you do something good, you're rewarded with dopamine and gain a pleasurable, happy feeling. This teaches your brain to want to do it again and again.

Serotonin is a neurotransmitter associated with memory and learning. Researchers believe it plays a part in the regeneration of brain cells, which has been linked to easing depression.
An imbalance in serotonin levels results in an increase in anger, anxiety, depression and panic .
Norepinephrine helps moderate your mood by controlling stress and anxiety.
Leave aside these biological engineering with which the biological robot called the human kind has been built by nature, and focus on the logical inference it leads to.
The logical inference takes us to the simple fact that emotions are involuntary actions that occur each moment in our day to day life . Our life thereby runs by default, so to say as long as the involuntary or 'by default' actions occurring within goes on unabated.
That's the core reason, why the human kind is a tormented lot, being a victim of fluctuating fortunes that actions or inactions impelled under the ongoing robotic designs carry the man.
That brings to the fire the biggest challenge the man kind faces. The challenge is to turn what is involuntary into voluntary. To turn 'by default' action of the mind,  into actions by will. By turning the involuntary into voluntary. By refusing to be mastered by emotions and to acquire mastery over emotions.
The key words, 'mastery over emotions' is what  brings us to the definition that various yoga forms give, Karm Yoga prescribed by Bhagwat Gita being the simplest ,even though projected by misconceived interpretations as the toughest .