system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What chemical reaction do drugs have (in particular MDMA) that makes us more sociable and why can’t people just “be like that” when they’re sober?

Does it remove that anxious social barrier or it something else? Really intrigued as to what goes on in the mind.
gpt
There are stuff in your body and brain called neurotransmitters. These are molecules that some cells secrete and some cells take up, this is how they communicate. The most notable are probably endorphines (your bodys own painkillers), dopamine (reward molecule, it gets secreted when you do something that is beneficial to you and your survival like high calorie food, sex etc) and serotonin (love molecule, it makes you attached to stuff and makes you feel happy and euphoric) 

MDMA, when you eat or snort it, goes into the bloodstream, then to the brain. In the brain, it makes cells secrete serotonin and dopamine, as well as norepinephrine (its like adrenaline, but while adrenaline is used mostly in the fight or flight reflex, norepinephrine is more broadly used by the body) this makes you energetic by activating the "oo stuff is happening better get ready" part of your nervous system. 

These chemicals if present, make you feel how i described in the first paragraph, except you did nothing that would normally get the chemicals flowing, its just the mdma molecules on the cells constantly supplying the information that they have to make the happy chemicals, so they make lots and lots of it, making you feel the high.

Another thing is that your body usually picks these up to get the concentrations of the neurotransmitter down to the resting level. Mdma also blocks this, so the already higher than normal concentrations keep being higher than nornal, making you feel happy for longer.