Haleivi: Not “only.” Obviously it made its way into tanach because the most important meaning for Jews in general is the relationship between God and knessess yisrael, but you can’t take a whole sefer of tanach and divorce it entirely from what the actual words say.
If you think “chasdei Hashem ki lo samnu” is a better example, use it gezunteheit. As for why I think shir hashirim was used, because saying things like that to your wife is holy. Saying them to a waitress is not.