Slannesh and his/her demons are entities of excess. It starts nice enough, with everything just seeming better; food is tastier, sleep more restful, beauty more dazzling... Then like a drug, you start to need more and more to get the same high and start delving into ever more depraved acts just to feel the ecstasy Slannesh promised you. Daemonettes are the epitome of that. They'll do things to you that cause you unimaginable agony and you'll like it, thanking them for the privilege as they slowly peel your skin off, fry it, season it with psychotics and neurotoxins and feed it to you.
"Look, I come from the part of the Realm of Pleasures that has higher gravity, you gotta be more robust over in that neck of the woods.... Skinny Britches types just collapse into a mess of organs and chitin. It's kinda hilarious."