A Hug
Somehow, the most abstract and intangible attachments you have to people still seem to remain dependent on (or at least effected by) physical experiences — from experiences as trivial as colocation to more substantial ones.
The single most painful way to be disconnected from someone you love is, I think, having never shared a hug with them and, for one reason or another, having no hope of ever doing so.
Silly-sounding, yet so resonant that I can’t dismiss it.