Lyric discussion by Cornflksandcheez 

Cover art for Where Or When lyrics by Harry Connick, Jr.

This is not just about Deja Vu. I think it's about reincarnation.

Loving someone so much but having little reason. Feeling familiarity with someone you don't know very well but know you feel comfortable with.

It remains a mystery where but the question "when" suggests that time certainly is a factor. And unless amnesia took place...it's probably referring to another lifetime.