Santosha literally means “contentment, satisfaction.” This word, and the concept behind it, is a strong theme in Yoga literature. With the goal of most yoga practices being composure or the ability to witness experience without being disturbed, contentment or something like it would be appreciated by many texts and traditions.
A hymn from the Rig Veda praises men who are close to Indra. The verses describe this well-rounded person who can fight when they need and be kind when that is appropriate. The verse below speaks of the ‘friend of Indra’ as dwelling in contentment.
Highest and lowest, men who stand between them, going, returning, dwelling in contentment,
Those who show forth their strength when urged to battle−these are the men who call for aid on Indra. ~Rig Veda Hymn XXV, Verse 8
In the Upanishads and texts of Vedanta, Santosha is stoicism and the practice of ignoring sense desires.
If you are seeking liberation, my son, shun the objects of the senses like poison. Practice tolerance, sincerity, compassion, contentment and truthfulness like nectar.
~Ashtavakra Gita Chapter 1, Verse 2
Though without wealth, he is ever-content, though without help he is very powerful, though he does not enjoy sense-objects, he is eternally content, and though without an equal, he sees equality everywhere. ~Vivekechudamani, Verse 545
Contentment is equated with wisdom. If the senses are controlled strongly enough, they will stop pulling the mind and it will become established in peace.
When he leaves behind all desires emerging from the mind, Arjuna, and is contented in the Self by the Self, then he is said to be one whose wisdom is steady. ~Bhagavad Gita Chapter 2, Verse 55
He who has abandoned all attachment to the fruits of action, always content, not dependent,
Even when performing action, does, in effect, nothing at all. ~Bhagavad Gita Chapter 4, Verse 20
Content with whatever comes to him, transcending the dualities [i.e. pleasure, pain, etc.], free from envy, constant in mind whether in success or in failure, even though he acts, he is not bound. ~Bhagavad Gita Chapter 4, Verse 22
In the Bhakti tradition, which has roots in the Bhagavad Gita, Santosha is trusting god and being content with what god gives. This is taught as both a practice and a final state of consciousness.
The yogin who is always contented and balanced in mind, who is self-controlled, and whose conviction is firm, whose mind and intellect are fixed on Me, and who is devoted to Me, is dear to Me. ~Bhagavad Gita Chapter 12, Verse 14
A person engaged in pure devotional service neither desires anything for sense gratification, nor laments for any loss, nor hates anything, nor enjoys anything on his personal account, nor becomes very enthusiastic in material activity. ~Narad Bhakti Sutras 5
Patanjali, with his strong Buddhist influence, is vague on the details of Santosha, but strong on the effects. His words point clearly to the Buddha’s teaching of suffering and attachment.
From contentment comes supreme happiness. YS 2.42
To Swami Kripalu, contentment needed to be practiced with discrimination. He spoke of situations where contentment was really stagnation. He taught that we need to do all the right things, then practice contentment.
Contentment is where an individual has a capacity or the ability to bear with all the difficulties and problems of life, and digest them appropriately.
Here one should not forget that the contentment of a worldly person is different and the contentment of a sadhak is different.
In one way, we can say that the contentment is very good. From a higher point of view, contentment is not that good either. So, dissatisfaction or discontentment is useful. And contentment, you know, is also useful in its right place.
Individuals who are progressing materially, they progressively remain discontented. He always intends to grow further and further. He is never satisfied with the condition which he is in. Similarly, when a sadhak continues to progress spiritually, he continually feels dissatisfied with his spiritual progress. He desires to progress further and further and he is not satisfied with the condition he is in.
All quotes above by Swami Kripalu are from his discourses and informal talks. Edited versions may be available in the Prem Yatras and A Sunrise of Joy: The Lost Darshans of Swami Kripalu, by John Mundahl.