The person noticing a change has been keeping some part of you in mind.

Noticing requires a before. When someone says you seem lighter lately, or asks whether the thing you were dreading happened, they are showing you that a version of you has been sitting somewhere in their head, being updated. That is a quiet form of love, and it costs something to maintain. Most people never mention that they are doing it, which is why it usually only becomes visible in the moment of noticing.

Think of who has done this for you recently. Then do it deliberately for someone else: remember one detail they mentioned last month, and ask about it.