The speaker starts with a strange request: don't believe me when I say we're best friends. Everything that usually proves a close friendship, the matching necklaces, the claim that I'd choose you over anyone, is offered and then undercut. The title makes this sharper. "BFFs" is the kind of shorthand kids use for forever-friends, while "Ancient Artifact" suggests something old, dusty, and already behind glass. The friendship is treated as a relic of a time when such promises felt real.
The speaker's reason is blunt: nothing lasts forever. The last three words, each on its own line, fall like a countdown. Love can be "faked," then "unrequited," then "empty." Each word is a different kind of failure, and the list moves from deception to pain to hollowness.
The mood is guarded and bleak, but also a little sad. Whoever is speaking seems to expect disappointment so firmly that they warn the other person in advance, perhaps to protect themselves as much as the friend.