Darkness here seems to stand for a painful inner state, something like depression or overwhelming anxiety, though the poem never names it. The speaker talks to it directly, as if it were a person sharing their head. At first the language is harsh: darkness is a "monster" that eats the brain, drinks the mind, causes troubles, and makes the head "burst." It has even turned the speaker into a "psycho."
Then the last lines flip everything. Because the darkness "never left," the speaker calls it a "best friend." That ending is the heart of the poem. It suggests a sad kind of loyalty: when nothing else stays, even something that hurts you can feel like company.
The mood is raw and bitter, with a dark, almost joking tone in the repeated "Yeah" and "Yep." The short, blunt lines feel like someone saying this out loud without dressing it up. The result is uncomfortable, but also honest about loneliness and how strangely we can cling to what torments us.