Getting high reveals how arbitrary the connection is between how you feel and how well things are going.

It refers to the fact that feelings are not a reflection of the outside reality, but a reflection of one's perception of it. According to OP, this is proven by how feelings completely change by simply changing the way the brain perceives reality, via a psycotropic compound, while actual reality remains unchanged.

This is a well known scientific and philosophical fact, that OP has only come to know recently thanks to personal experience with psycotropic drugs

Such epiphany resulted in the shower thought we are commenting.

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It doesn't really matter what's happening, with regards to how you're feeling. You can be going through shit and having a good time, or king of the world and just miserable.

I assume they refer to smoking weed. It can show you the mountain before you is not always high, and that it is not always a mountain.

Sober, you might feel completely different about some specific problem, but with this you can actually take a look at it and deconstruct the problem in peace

It about how much you are influenced by chemicals made by your body in your day to day life.

Like how much your emotional state of mind depends on your body chemistry going perfectly.

Getting high on weed gives you a noticeable, controllable disruption to highlight this effect.

Weirdly enough I'm high AF and it makes sense to me

In my experience, it made it very clear how easy it is to have a disconnection between what is happening in the "real world", my perception of it and what happens in my brain.
Basically your brain is a big box of chemical reactions that happen regardless of what's going on in the world, and you unconsciously interpret the world through whatever your brain makes you feel at the moment. (For example, think about the fact that you don't notice your nose most of the time, it's there at all times but your brain filters it)