How are serotonin, dopamine, endorphins, and enkephalins related? Also what's the function of each on its own?
Catecholamines are epinephrine, norepinephrine, and dopamine [1]. End: Epi, NE, DA is involved in reward circuits [1]. Serotonin is involved in depression and aggression [1]. Depression can also be triggered by a dearth of endorphins [2]. Drugs can also trigger the production of serotonin, leading to drug addiction [2]. Serotonin is produced after eating carbohydrate-based foods, such as pasta and grains and released can produce calming effects and a sense of comfort and wellness [2]. With a dearth of serotonin, however, stress, anxiety, frustration and aggression can result [2]. The function of each of these neurotransmitters on its own is as follows: serotonin is involved in depression and aggression, dopamine in reward circuits, enkephalins in arousal and endorphins in calming [1].  
Dopamine motivates us to take action toward our goals, desires, and needs, and gives a surge of reinforcing pleasure when achieving them. Studies on rats showed those with low levels of dopamine always opted for an easy option and less food [1]. People with higher levels exerted the effort needed to receive twice the amount of food [1]. However, happy endorphins didn't rise to fame because they block pain signals, but because they induce euphoria [4]. That's right, the ubiquitous "runner's high" is caused by a big rush of endorphins [3,4]. Endorphins are hormones and neuro-signalling molecules that function as painkillers. They inhibit the transmission of pain signals in the central nervous system by binding to opioid receptors (the body's natural morphine) [4]. Endorphins and dopamine are often confused because each one is a chemical that makes you happy in the broad sense of the term [5]. serotonin is also called the happiness hormone, dopamine is a feel-good hormone, and oxytocin is the cuddle hormone [5].