As Jeff sang loudly in his car, he realized that the
drivers nearby were staring at him.

Then Jeff is crazy and has mental issues and has schizophrenia

Explanation:

Jeff with schizotypal personality disorder have difficulties forming relationships and experience extreme anxiety in social situations. They may react inappropriately or not react at all during a conversation or they may talk to themselves.

Talking to yourself isn't just normal, it's good for your mental health — if you have the right conversations. “Talking to ourselves is completely within the norm. ... In fact, we talk to ourselves constantly.

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