I dunno if this will thoroughly answer DF's questions (however poignantly asked...as I've pondered much of the same things), or..for that matter...ANYONE'S questions...but this is how I see it:
Love is fathomless. We truly do NOT have dominion over love and its fathomless, most ambiguous of meanings. Capturing it is difficult enough...defining it is tougher. But if you think about it, love is a rather primordial emotion. We love our family, we love our closest friends, we may have even loved one of our teachers that really made an impact on us at one point or another. If you ask me...the REAL challenge is-- do you LIKE a given person? What is it about them you LIKE?? Do you like that person enough to want them around day in and day out?? I'm sure a healthy number of us have said something along the lines of, "....I dunno...I love (so-and-so) and all...I don't wish him/her dead or anything...but right about now, I don't like him/her." So...does this mean that liking someone is a bit more important than loving them??
We feel the burn of losing one we love (be it by death, a breakup, or what have you) because of the time and energy we invested in that person. Yes..it may seem like a futile, hopeless waste of time when we are grappling with the emotions of having lost the one(s) we love(d)...but what keeps us coming back for more (I think) is what our souls resonate to our conscious minds. It is our souls that yearn to feel and give the love we're capable of administering...especially when we've had the qualitative time to grieve and get over the loss of a love we once enjoyed. Think about those among us who lost a spouse. As time goes on (and its time that varies from individual to individual), the one who survived the loss could find themselves in love again and could marry again for it. Does that mean they've forgotten the one they loved and lost? Of course not. The memory of that person is very much alive and well. Often we hear people lament, "...I'll never love another person again the way I loved so-and-so..." Well, of course you won't. The next person you fall in love with (or find love with) will be a whole 'nother individual--with their own personality, humor, character and idiosyncrasies; the love given that person in turn will be different in dynamic and energy from the way they felt 'love' for the person they lost...and yet, if the relationship is good and complete...and they LIKE that new person...it will still have a feel of completion. It's our souls that tell us whether or not that completion we seek has been found....hence the answer to the age old question, "How do you know you found it!??"