This is why they fuck with me (real talk nigga, believe it)
This is why they fuck with me (do what I do, y'know?)
I'm no gangster, no killer, I'm just your average Joe (know that)

from Kendrick Lamar – Average Joe Lyrics on Rap Genius

Meaning

Kendrick is just being himself. He’s not different than any of us, he’s just an average dude who keeps it real.
This has 2 meanings. this is why the fuck with him, like this is why he gets hassled by the gangbangers and stuff. Predominantly, though he is saying that this is why they fuck with him, as in this is why people like him and his music.
They fuck with him cause he remains real and himself, never selling out for an image.

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