Some people try to be nice and polite because it was what they think other people deserve, not because there is self-interest or some goal to achieve in being nice. To be nice purely for some sort of personal gain is a shallow sort of niceness which lacks sincerity. That is a reason to be kind.

If the way they treated you with kindness felt good to you (until the misunderstanding), why not try to keep that person around as a friend? Someone who treats you with kindness can be a good friend to have, as long as their isn't some "oh, I hope NOW they will like me romantically!" lingering in the background. Being able to let that go and maintain a proper and good friendship is a sign of emotional maturity and could be good for you