Te argument for mu-opioids such as endorphins as your happiness/pleasure system is supported by detoxing opiate addicts in withdrawal — and I have personal experience. One’s endorphin/enkephalin/dynorphin (or mu-opioid) system finds itself with lower signaling strengths because those receptors are lacking, having been either desensitized or removed (down-regulated) during the flood of opiates thrown at them, a form of reversible brain injury. One psycho-behavioral effect is the renewal of old urges to perform some action that FEELS GOOD. Suddenly, I was eating cookies by the fistful and craving cigarettes (I quit 30 years ago).