Part of the brain creates a neurotransmitter called serotonin. Serotonin binds with the opioid receptor, too. Its job, when it does that, is to promote satiation, contentment, happiness, patience—telling you you’ve had enough, you’re fine. Our brains have evolved to naturally keep serotonin and dopamine in competitive balance. Dopamine tells us that something better awaits us; serotonin says we have enough.
