How trying to love 'all' humanity can be selfish.

The novelist Jonathan Franzen puts it like this:

Trying to love all of humanity may be a worthy endeavour but, in a funny way, it keeps the focus on the self, on the self’s own moral or spiritual well-being. Whereas to love a specific person, and to identify with his or her struggles and joys as if they were your own, you have to surrender some of yourself.

In a broader sense, the more concrete notion of the common life of a neighbourhood or town is something that really exists and can grow or shrink. Similarly the idea of a common purpose for a group or a whole country, or the whole of humanity is a contestable idea.