But sometimes a rocket up the arse IS being kind. I don't believe it's kind to tell gender questioning people they can change sex, or tell obese people there's nothing they can do about it because their genes make it impossible for them to stay slim, or a myriad of other similar things which often get people shot down on here.
Kindness is saying these truths in an appropriate and understanding way (only when asked for an opinion obviously)! Unkindness is telling easy lies which don't help the person in the long term but create less conflict for the lie-teller.