For me, personally, I know that you could choose a person at random in the world, write a paragraph about them, and give it to me, and by doing that, I would care about them a lot more than before I had read that piece of paper, even though reading that paper hadn't changed anything about them. Similarly, becoming friends with someone doesn't usually change the person that much, but increases how much I care about them an awful lot.

Therefore, I look at all 7 billion people in the world, and even though I barely care about them, I know that it would be trivial for me to increase how much I care about one of them, and therefore I should care about them as if I had already completed that process, even if I hadn't

Maybe a better way of putting this is that I know that all of the people in the world are potential carees of mine, so I should act as though I aready care about these people in deference to possible future-me.