system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why you get the munchies when you're high.
gpt
The human nervous system have specialized receptors for the chemicals in marijuana (The two most people study are THC and CBD).  These chemicals also effect the neurotransmitters Serotonin (5ht) and Dopamine (DA).  Marijuana causes a burst release in both 5ht and DA.

The part of your nervous system that controls appetite is the hypothalamus.  It controls "appetitive behaviors", or what we call the "four F's" - Feeding, Fighting, Fleeing, and Reproduction.  

All three chemicals effect the feeding part of the hypothalamus (CB1 from Marijuana, 5ht and DA) resulting in increased levels of percieved hunger.  It's often used in treatment for people with depressed appetites (chemo patients and the like).

Just so you know, the THC component is what gets you high, the CBD components usually have the medicinal effects.