There are grossly three main parts of us - Intellect (brain) Mind (Mann) and body. Then there is an external world. If you purely listen to your brain, you are less likely to be unhappy. Mind creates desires. Some of them are satisfied by the external world and some are not. The unfulfilled desires lead to unhappiness. Also when one desire is satisfied, you become neutralized for that desire. e g as a kid you crave for two wheeler like anything and when desire is satisfied, after some days it just remains as a vehicle for you. You did not get continuous happiness out of that.
It is extremely important for your intellect to have control on your mind and body. Use body as a vehicle. Ensure maintenance and petrol fuel. But treat it as your slave and you don't become slave of your body. Mind creates desires. All of them are not granted by the world. There is constant struggle between mind and world. This reduces the energy level of the person. e g after 8 hours work you feel tired. If you actually calculate the time and efforts spent - physically and mentally on the task, then you will realize that you cannot justify the tiredness. Still you are tired. That is because of the struggle between your mind and the world. Your mind wants something, world behave differently. You have no control over the world. But you can control your mind. The energy wasted in the struggle between the world and the mind can be used by intellect to do proper act. If you use that energy in working/action instead of struggling, you will be more productive. Desires are endless. Your intellect should control them to ensure higher productivity and efficiency.
( Learning from Mr Parathasarthy )