It’s not that they care; everyone should care about everyone. Instead, it’s because they know more about a person that they already care about.
You can look toward a stranger, care about their life, sacrifice your life for it, even. That’s because you know that when a person is falling, they’ll only ever be fulfilled if you catch them. That’s true of anyone.

But if you know more than that, a lot more, things get messy, since you get a much more complex idea of what their fulfilment could look like. If you see your friend on a path to fulfillment, their fulfillment, you support them. If they’re on a path that you don’t think leads to their fulfillment, it’s more complicated, but you still support them.

You support them, because if they continue, they’ll need it, and if they change paths, they’ll need it. But that support, if you don’t think they’re on an in-the-end-fulfilling path, won’t be a pair of hands pushing them forward.
It’ll be a pair of hands holding them up, with a slight backward pull to keep their progress on that path from getting away from them. But your hands are mostly just holding them up.

Do you think you know their fulfillment better than them? The direction of your hands’ force depends on that. Do you push forward strongly? Do you hold up and slightly push? Do you hold up and slightly pull? Do you yank back from an edge and shake their shoulders, snap them out of it?

This all relies on your being self-aware, knowing what you truly know about your friend’s path to fulfillment. The better the friend, the closer the friend, the more you know, the more you need to be self-aware. Yet also the more you can push before your hands will be shaken off, and perhaps rightly so. No matter how close you are, sometimes, not always, but sometimes, a person knows their own fulfillment like no other.

A good friend always supports but, knowing what they truly know, rarely shoves. A good friend is a constant, gentle force. Perhaps it’s not in the direction you yourself want or expect, but it’s gentle, and it’s for you.