Because grinding leads to…
Strong hands on your hips and getting to wriggle alllll you like!
Update: Am I a very bad man for thinking how much fun it would be if you were wriggling like that because you really, really needed to pee and I wasn’t letting you?
I’d never be so mean as to be inside you when your bladder was really full (such an awful word, “bladder,” no?) But grinding against me like that when you really had to go would be kind of marvelous!
What do you think, guppy?