Concerned with the mind, the pure nature of mind is to be at peace, that is to have only one thought or a homogenic thought. When one is happy, one has only one homogenous thought. This state may be more a few seconds and hence happiness is also not ever lasting. Doubts and fear gives rise to many thoughts not pertinent and hence one becomes sad or depressed. The purification method of the mind is through creating homogenic thoughts or to bring in thoughts which the mind is comfortable of.
Meditation is method of purification of the mind.
In meditation one is trained to bring homogenous thoughts and so one enjoys happiness or peace of mind. The meditation method is taught in steps. In the beginning steps the mind is made to relax and to slowly observe ones thought as one observes how his hand or leg moves. In meditation one objectifies one's body organs and the observer becomes the subject. This is very simple.