When you are using a highlighter, you may become "highlighter happy."
This means that you when you put your highlighter down to highlight, you cannot stop drawing. Soon, your whole paper may be colored with drawings and brightly colored lines.
When you take a shit and you wipe your ass so many times and it just doesn’t stop staining the toilet paper, your asshole then becomes a highlighter due to the amount of wipes it takes to flawlessly clean your asshole.