I've long thought of being nicer as coming in two flavors: increasing how nice you are to those you don't like (love your enemy" and increasing, as you put, the net of niceness (classifying less people as your enemy).

That of course has a very simple view of niceness: you view people in only two categories. But I think it generalizes nicely to the nuanced groups people interact with: make the core group bigger or just be nicer to everyone. Of the two, I think the second is the hardest.