सत्त्वशुद्धि-सौमनस्यैकाग्र्येन्द्रियजयात्मदर्शन-योग्यत्वानि च ॥४१॥
41. There also arises purification of the Sattva, cheerfulness of the mind, concentration, conquest of the organs, and fitness for the realisation of the Self.
By the practice of cleanliness, the Sattva material prevails, and the mind becomes concentrated and cheerful. The first sign that you are becoming religious is that you are becoming cheerful. When a man is gloomy, that may be dyspepsia, but it is not religion. A pleasurable feeling is the nature of the Sattva. Everything is pleasurable to the Sattvika man, and when this comes, know that you are progressing in Yoga. All pain is caused by Tamas, so you must get rid of that; moroseness is one of the results of Tamas. The strong, the well-knit, the young, the healthy, the daring alone are fit to be Yogis. To the Yogi everything is bliss, every human face that he sees brings cheerfulness to him. That is the sign of a virtuous man. Misery is caused by sin, and by no other cause. What business have you with clouded faces? It is terrible. If you have a clouded face, do not go out that day, shut yourself up in your room. What right have you to carry this disease out into the world? When your mind has become controlled, you have control over the whole body; instead of being a slave to this machine, the machine is your slave. Instead of this machine being able to drag the soul down, it becomes it greatest helpmate.
- Swami Vivekananda
The Complete Works of Swami Vivekananda/Volume 1/Raja-Yoga/Patanjali's Yoga Aphorisms - Concentration: Its Practice