Why does it feel like the most caring people are never anyone's "best friend"?

My friend told me recently that she has another friend who she treats as a high priority but she knows that friend doesn't see her as a high priority. It made me really sad because my friend is a wonderful person and caring individual who always shows up for the friends in her life.

Why does it feel like the most caring people end up feeling the "These people are my best friends, but I'm none of theirs" sentiment? Why can't people who care find each other?