Development of Sattva is not only important for a Yoga student, but it is the main activity in Yoga practice. The main purpose of Yoga practice is to develop mental focus that can be used for one’s goal, mainly spiritual goal. Yoga deals with the mental focus and that needs one to have an uncluttered mind. Uncluttered mind means Sattvic mind. However since body and mind are intimately connected in the normal daily activity, Sattvic mind needs Sattvic body also. Very muscular body or very fat body both lead to lack of mental focus. Body is made from the food. So, food has to be Sattvic as well. Similarly, the type of mental engagement has to be Sattvic, otherwise mental focus gets distracted often.
When an activity of Sattva quality is repeated, it forms an impression inside the mind. This later makes one engage in Sattvic activity, thus increasing the total Sattva. Repetition is the key in developing Sattva.
In order to know how one’s Sattva is developing, one should observe the nature of his/her diet and resulting body. Also, one should observe what kind of thoughts are passing through mind on an average. Try to develop Sattvic thoughts repetitively.
Once a predominance of Sattva is developed, it will drive a person further only in Sattvic activities. This predominance is called Sattva Shuddhi, means sufficient Purity of Sattva. When purity is complete, one has reached the goal.
sattvapurushayoH shuddhisaamye kaivalyam… Last stura of 3rd chapter of Yoga Sutras of Patanjali.