The mental health profession is focused on the brain chemistry, assuming it's the be-all of brain health. Psychedelics on the other hand focused on experiential healing, proving that limited experiences can have permanent effects on the brain. 

"Can you achieve the same results through talk-therapy or meditation? Sure ... But it will take you 10 years. With psychedelics it took me an afternoon!"

The default mode network is a group of brain regions connecting the cortex, the rational new part of the brain & the older part focused on emotions& memories. It is believed to play a large part in our ego, because it rests when we are focused and active when we are awake and not involved in any specific mental exercise like:

• self-reflection
• thinking of the future

Quieting the default mode is what happens through meditation or psychedelics. 

Why is that psychedelics work on rather distinct mental problems like depression, anxiety and the addictions associated with each? 

A hypothesis is they are all symptoms of the same problem - obsessive thinking (too much ego). After all:

• depression is regret about the past
• anxiety is regret about the future