So I think we start developing these synapses for parents and significant others when we're children. There is a way of feeling that only my Dad could evoke in me, and my mom, and my brother. No one else could make me feel those subtle levels of love, comfort, fear, or disappointment. (I feel certain I've tried to get people to stimulate those receptors, and have made poor relationship choices as a result) That might explain the acute pain of someone leaving or dying; parts of the brain are actually starving. I called my Dad's recent death a partial soul-ectomy, because it did actually feel like something was being torn from my being, but the starving receptors model would fit too.
And friends. I have a handful of friends who have loved me and been loved by me for more than two decades, and/or with whom I have lived through unusual and bonding experiences, and those friends now have their own receptor sites. These sites hunger for them at times when they aren't being stimulated, and pulse and thrive when they are. An anemic word for this feeling is love, but it is more than that. More nourishing, fulfilling, enriching than just one word could encompass.
Perhaps loneliness is simple brain science. Maybe in the future we can provide electrode-stimulation for our loved ones so they won't miss us when we're gone. Although with that replacement for yearning, our songs and other art forms would all suck. I don't think I'd be the same person if I'd never heard Bob Dylan's "Tomorrow is a Long Time" . What beauty in these words:
Yes, and only if my own true love was waitin',
Yes, and if I could hear her heart a-softly poundin',
Only if she was lyin' by me,
Then I'd lie in my bed once again.