Well, if you must know, it was because he was very jealous and I had these days-of-the-week underpants.
6 seconds sound clip from the When Harry Met Sally... movie soundboard.
You can hear this line at 00:09:40 in the Blu-ray version of the movie.
Quote context
[...]
- How do you know we broke up?
- First of all, I am not with you.
- And second of all, it is none of your business why we broke up.
- You're right, you're right. I don't wanna know.
- I'm sorry, I need a judge's ruling on this.
- Days of the Week underpants?
- Yes. They had the days of the week on them and I thought they were sort of funny.
[...]