Happy and pleasant state helps in total equilibrium of body and mind. In such state even Endorphins come into action and refresh the neuronal structures, so such person can achieve the near most state of Intelligence expression. Exactly apposite to this is state of sadness and anger, where neuro-chemical secretion either makes the neuronal functions dull or throws it in chaos. Then even the person with higher intelligence expresses weird results.
Indian Philosophy states the optimum Intelligence Quotient as, the IQ which serves the intellectual purpose at it's highest level, which in tern beneficial at personal level as well as universal level. Even those intellectual inventions which are serving us{humans) but are harmful to Nature are not supposed to be optimum intelligent expressions. Likewise happiness, balanced state of mind and satisfaction are pre requisite of optimum IQ and are by products of same too. But talking about day to day routine situations even pretension of happiness, fake smile on face at least work at superficial level and serve the purpose of at least bit improvement in IQ.
So sometimes it is said by the Wise People that" at least pretend to be happy and force a smile on your face, rather than playing with blues and anger and jealousy. Because comparatively it is useful to you and others too. "
