Her 5yo got it right.

By default you should be nice to people. Care about their needs, avoid offending them (and if you do, apologise), avoid unnecessarily lies, all that thing. And since most people are surprisingly reasonable, they’ll do the same towards you.

However. There’s always “that” arsehole, petty, assumptive person. No matter how nice you are towards them, they won’t be nice to you. Don’t be nice to them - even if on the outside it’s irrelevant, it’ll eat you from the inside and make you feel like shit.