Every niyama in the Yoga Sutras carries a stated fruit, and the fruits escalate: purity yields fitness for vision of the self, austerity perfects the body and senses, study brings communion with one's deity, surrender brings samadhi itself. Against that company the fruit of contentment at sutra 2.42 is startling: santoshad anuttamah sukhalabhah, from contentment, the attainment of unsurpassed happiness. Not a means to happiness, and not a modest happiness: unsurpassed, the superlative the sutras otherwise reserve for the summit of practice. The commentators take the grammar seriously.
Happiness obtained through objects, Vyasa notes, is always borrowed and always taxed: it depends on acquisition, is shadowed by the fear of loss, and ends in the craving for repetition. The happiness of contentment is structurally different because it is not obtained through anything; it is what remains when the machinery of wanting falls quiet. That is why it cannot be surpassed: any happiness produced by getting something is bounded by the thing got, while the happiness of needing nothing has no such ceiling. The teaching is easily sentimentalized, and the tradition resists that by making santosha an observance, a niyama, something practiced rather than felt. One rehearses sufficiency, daily and deliberately, in a mind trained since infancy to rehearse lack, and the sutra's promise is that the rehearsal eventually becomes the performance.