What is the difference between complacency and contentment (santosha)?

In the practice of yoga, santosha (contentment) is one of the niyamas, or disciplines pertaining to our own self governance (in contradistinction with yamas which are disciplines pertaining to our interactions with others).

It is defined as disinterest in accumulating more than one’s immediate needs in life, for “the entirety of whatever there may be within the entire world to satisfy one’s senses cannot satisfy one whose sense are uncontrolled…A person who is not self-controlled will not be happy even with possessing the whole cosmos [Therefore] one should be satisfied with whatever he achieves by his previous destiny, for discontent can never bring happiness.”—Vamandeva Avatar to Bali Maharaj

Even if there is some lack, the discipline of santosha is the power to say, “This is enough!”

Santosha is often juxtapose against desire (kama) whose nature eclipses self-knowing [Gita 3.38], is “never satisfied and burns like fire” [Gita 3.39] and is (therefore) the enemy of the self [Gita 3.37]. Opposed to desire, santosha is a strategy for encountering human limitation created by daiva (destiny) or karmic-inheritance which manifest as a type of birth, lifespan, and life experiences [Yoga-Sutra 2.13]. Although it’s not possible to empirically ascertain the limit of the limiting imposition on human life (daiva), santosha (and a general mentality of less is more) insures one doesn’t desire beyond the constricting circle, lest one spoils the peace of their inner life with discontentment by trying to capture the moon; thus making happiness an impossibility [Gita 2.66].

The main difference between santosha and complacency—defined as a smug and uncritical satisfaction with oneself or one’s achievements—is that santosa is purposed for the attainment of a great human achievement, viz samadhi (self & God enlightenment). Complacency, on the other hand, arrest human achievement, even in the conventional domain, what then to speak of the spiritual one.

Santosha is about quelling the passion for the external world, thus acting as a fertilizer for the seed of inner potential causing it to blossom and manifest, in time, the flower of enlightenment and henosis (unification with divinity). Complacency, like arid soil, makes impotent that seed of inner-potential and inevitably ends up in the land of discontent and suffering just like unhinged desire.

Said simply, santosha is purposed for abiding happiness; complacency is purposed for abiding suffering.