In plain words
The reason, they said, is that this laughter is a cause of sorrow. There is one laugh, and that is the smile. It makes no sound. The teeth do not show. A laugh that makes the face blossom from a pure heart. That smile alone. Our duty is to do deeds that make that smile leap and dance. Some crack a few jokes and keep everyone laughing. With this, the whole of life takes on nothing but the form of a joke. So, as for our thoughts: we must foster the thoughts by which we smile and, through us, make others smile. To lead such a life is man's chief duty. This is found only on the spiritual path and on no other path. They speak of the fourfold antaḥkaraṇa. The antaḥkaraṇas are four. Mind, Buddhi, citta and Ahaṅkāra: these are the forms of the antaḥkaraṇa. In this fourfold antaḥkaraṇa, the very first is the mind. So the mind alone holds the reins for all of them. When we keep the mind under proper control, our life is blessed.
By the mind births arise again. The mind is the cause of our taking many births again. By the mind births vanish away. The mind is also the cause of becoming free of birth and having no birth. By the mind the four called Dharma, artha, kāma and Mokṣa, the fourfold puruṣārthas, also find their fulfilment. By the mind steadfastness in yoga shines. By this mind alone all the yogas we practise, karma yoga, Jñāna yoga, Bhakti yoga, rāja yoga, this kriyā yoga, reach fulfilment. So in the Vedānta, which holds the mind to be the cause of bondage and Mokṣa, they said, "Manayeva manuṣyāṇāṃ kāraṇaṃ bandhamokṣayoḥ." The mind is our life. The mind is our breath of life. The mind is this world. The mind is substance. Without this mind, there is no tumbler here. Without this mind, there is no mike here. Without this mind, there is no table. All objects are nothing but forms of the mind. So everything has arisen from the mind alone. So we must fill this mind with pure thoughts. All consciousness is held within this mind alone. If we make this mind broad, it takes on the form of God. If we drive this mind down to ruin, then we fall in every way. The more we grow this mind, the more and more it develops the sacred path. So we must set the mind on the right path. We must see to bringing this mind back to the place it came from.