Happiness and sadness are state of mind. We feel happiness because we 'think' and 'believe' something good has happened and vice versa we feel sad when we think something bad has happened.
The reason for one's happiness may be the reason of someone else's sadness. Very trivial example - victory is a sport: the winner is happy and runner up is sad at the same time, with same outcome.
Coming back to the point, once we understand we are happy, there comes satisfaction and bliss which leads to more happiness and we stay happy or more happy, till the state of mind stays. Once that state of mind is gone, we are back to normal, mundane life.