At its core, being nice is about being liked by others by making everything smooth. No waves, no friction. It’s based on this (woefully inaccurate) theory: If I please others, give them everything they want, keep a low profile, and don’t ruffle feathers or create any discomfort, then others will like me, love me, and shower me with approval and anything else I want (promotions, sales, friendships, dates, sex, attention).
This theory is bunk. It’s an inaccurate map of human relationships. And like any inaccurate map, if you follow it, you will not get where you want to go. You will be lost.