Dismissed this thread initially, but then I got to thinking. Because I hadn't been previously.
I think love is special because it's both exclusive and temporary. If every single person on the planet had the kind of love outlined in this thread, it would be just another every-day part of life. Instead, we're all fully aware of the fact that a significant portion of the population doesn't have it - and a number of these probably never will. Further, we all know that love can be torn from us, irrevocably, by death or betrayal at a moment's notice. That sort of impermanence forces you to live in the moment, intensifying your feelings.
So to sum up: when you look around and see a lot of people without love, or a lot of people who've lost it, you can't help but intensely appreciate it when you've got it.
Also, there's some nagging idea, way in the back of my mind, that recognition of yourself in another reinforces basic existential concepts, which frequently have limited grounding outside of one's own mind.