Human blood contains receptors which receive signals from the brain. So, in a happy man, the receptors receive a signal of happiness from the brain and pass them. Then the process of healing is made faster. Hence, the more you laugh, the healthier you become.
By smiling, laughing and wearing cheerful expressions, we strengthen the immune system to prevent and fight diseases. If tension, stress and anxiety make a man sick, then happiness, cheerfulness, contentment and joy should help a man cure his diseases.
As for old age, "It is a state of mind. If a man's mind continues to be fresh and accept ideas he is not old. If hope lives within his heart and he continues to look ahead, he is not old. If the spark of love lives within him and he continues to expect the best from life, he is not old".
People today need a sense of humour to laugh often. Develop a sense of humour, a humour which brings blessings to oneself others and not the kind which will hurt others. Learn to laugh at yourself. It is easy to laugh at other's weaknesses but you must learn to laugh at your own. Always look on the bright side of things. Feel joy in everything that the will of god brings to you. There is meaning of God's mercy in everything that happens. Always be positive in your attitude towards life. A man is his own friend and his own enemy as none outside can do any harm. If you are happy, make others happy. The happiness that goes out of you will come back to you.
So, "Laugh and the world laughs with you".