Most of the unhappiness come out of our inability to manage expectations. Also, we are exhilarated when we have success while feeling extremely disappointed at failure. Somehow, most people are not in a position to handle fame and tragedy with equanimity.
Bhagawad Geetha tries to address this by talking about doing one's 'Karma' or duty without expecting any returns. By exhorting people to not expect returns, Bhagawad Geetha protects them from the feelings associated with success or failure.
There is a nice Zen Story on this..................
Once upon a time, a farmer had a horse. One day the horse bolted. The farmer's friends came to him to commiserate with him on his bad luck. He listened to them but just said 'may be'
The horse returned a few days later with 6 wild horses in tow. Everyone congratulated him on his good luck now. He simple said, 'may be'
A few days later, his only son fell down one of these wild horses and broke his leg. When the farmers friends talked about his misfortune, he said 'may be'
There was a war going on. A few days later the Army came in to take away all the able bodied young men to fight, but they left the farmers son back as he had a broken foot. When his friends gathered to talk about the farmers son's luck, he again just said 'may be'
Some might argue that the farmer's behaviour borders on fatalism. But if you look deeply, you will find that this is not passivity. The farmer is not judging events. He is accepting them for what they are. Sometimes it pays not to brood over what has happened, but stoically accept it and move on in life. This will also ensure that we manage our expectations.