The details you remember about someone become the proof they mattered to you.

Caring about somebody shows up as detail. You do not decide to remember that your friend hates the smell of gasoline or that their sister is called Nell; it happens because you were actually there when they said it. Years later those small facts are the evidence, which is why being remembered properly feels like being loved.

The useful half of this is that detail can be given back. Mention something small to somebody this week that they told you months ago and have forgotten telling you. It is a strange thing to receive, in the best way, and it takes one sentence.