I can only relate a few reasons of my own:
As repayment for her buying my dinner.
She said "Don't cum in my mouth"
To show them that you love them more than the other girl in the 3-way.
I was approaching climax, and no tubesocks were within reach.
You like the way she looks with her makeup off better than on.
She's wearing a dress she wants to return the next day. (Look for the price tag, It's a sure sign)
Because, for six months, around the office, she's been nothing but nice to you.
Her tits or ass are/is too beautiful to unload on.
You really like her alot.
You've always wanted to combine your favourite girl with your favourite bodily secretion.
Apologies to David Letterman