"Every time I close my eyes, I wake up feeling so horny."
Jermaine Dupri sat in the studio and looped a single chord progression until it felt like regret. came out of that repetition -- a song about wanting someone so badly it embarrasses you to admit it. Usher sang it like he was confessing to a friend, not performing for the radio. That's why it hit.
Number one on the Hot 100 for six weeks. The sound of a man singing exactly what he felt, no armor, no second takes.