[M4F] Why is it that one of the hottest sentences is "I can't believe I'm doing this"?

That's not a rhetorical question! I'm actually curious about why this is so. Close relatives of this sentence are "I can't believe he/she's doing this" and "I can't believe you're doing this".

What is it about being surprised by the lack of your own or someone else's boundaries being where we thought they were that's so irresistibly hot? Drop me a line if you have a hypothesis. Definitely drop me a line if you have an example to share or a fantasy to explore.