What conclusion can one draw by listening to the song "Sick Boy" by The Chainsmokers through the psychoanalytical lens? How could this be written in one sentence?

A conclusion in one sentence: The "I" in "Sick Boy" by The Chainsmokers stands for the American psyche, which the speaker critiques as filled with narcissistic projection and hysteria that desensitizes, feeding on individuals like the speaker while, ironically, labeling them as "sick"—but we can resist the disease. (This is a suggested conclusion: please work with this conclusion to make it your own.)
To look at a piece of literature through a psychoanalytic lens means to analyze it using the tools of modern psychology. This song lends itself to that method of interpretation because it uses words from psychology, such as "narcissism," "projection," and "hysteria."
In psychological terms, a narcissistic person is extremely self-centered, has an inflated sense of his or her own importance, and harbors an excessive need for flattery and approval. Projection, another term the writer uses, means assigning one's own feelings, especially unwanted feelings, to another person. Hysteria means having violent emotional outbursts and the symptoms of a disease with no underlying physical cause.
When the singer speaks of others projecting narcissism and hysteria onto people like him, he is saying that people with an inflated sense of importance take their own sickness and attribute it to people like him. He also says they desensitize others through hysteria or excesses of violent emotion.
In the fourth stanza, the singer states, "And they call me the sick boy." He says this ironically (irony is stating the opposite of what you mean). In other words, he is saying that the people who really are sick because they "spin lies into fairy dust," "feed" themselves with his "life's work" (are selfish parasites), and are indifferent to what happens to others project these traits onto him and call him "sick."
Although the part of America which is sick with narcissism and hysteria tries to project this onto others, the songwriter advises them—and himself—not accept the label. This assertion of a healthy self occurs in the fifth stanza, where the singer states:

Don't believe the narcissismWhen everyone projects and expects you to listen to 'em

The singer is saying we live in a sick society which tries to project its sickness o to us, but we can resist this. He ends on a note of bitter irony, singing:

Yeah, they call me the sick boy