Nicecore is a form of entertainment [showcasing] from the protagonist interacting with other characters in a story with [altruistic] actions and a optimistic view on others and the world around them.
While they may already have or, end up doing so for others throughout the story, the protagonist inspires them to do the same: just be nice.
Being nice no matter what. Because we never know what someone else is going through. And if you can be that shining light for them - even in a brief moment; of holding the door for others, letting someone go ahead of you in a check out line who has fewer items than you, saying 'Thank you're with a smile, or even just a smile - those seemingly "insignificant" moments can change someone's day. They can change the world.
Even if someone doesn't reciprocate, don't let it [change you]. Move forward and continue on your mission to be the one to change someone's else day when the moment arises.
[Altruism] isn’t a passive force. [Mister Rogers] used to say, “Love isn’t a state of perfect caring, it is an active noun like ‘struggle.’ To love someone is to drive to accept that person exactly the way he or she is, right here and right now.”