Thousands of candles can be lighted from a single candle, And the life of the candle will not be shortened. Happiness never decreases by being shared.

The candles purpose is to be lit and produce light. There are some ornamental candles that serve as mere pieces of decoration. But for the most part, candles are seen as sources of light. The paradox is that, when a candle burns to produce light, it consumes itself and ultimately 'dies'. Yet, in its death, it's purpose in life is achieved.
The other unique thing about a candle is that it has the strength to light anything else - even another candle. And like most other fires, by lighting another candle, it does not hasten to death or get consumed faster. It simply helps to amplify it's own light, and depending on what it lights, can even help achieve a million other things like cooking, decoration, burning and more. In fact noone lights a candle and hides it under a cover!
Joy and happiness are like that. If they are covered, ie kept selfishly inside, and never shared, the purpose is lost. By sharing your joy and happiness with others, you just add to their happiness and amplify the joy overall. It does not decrease the source of happiness inside you. Does a song make you happy? Sing it to others. You will infact find that spreading joy increases the amount of joy you feel inside and you will end up happier.
So be like that candle. We are all mortal and will one day pass away. During that time on earth, let your happiness light many others lives!