Offline for a few days with the holidays and very fun discussion! Thought to clarify some attachment related points. First, attachment theory in adults is a thorough field of study, only by other authors (eg Allen Schore, Dale Fonagey, Dan Siegel, and others) although it is not a universal carryover from early work on attachment by Bowlby, etc. The neurobiology of attachment is a study of how this relates to brain functioning and is very cool stuff, a google search on "mirror neurons" turns up lots of fantastic and interesting reading about the interconnection of humans, adults, and how connection can be neurologically "mapped."
And yes, only a theory. For myself, in combination of study and personal experience, I would consider attachment to be more in line with an "instinct" rather than a "need" (because most of our hearts would continue beating and we would survive in the absence of others.) Humans are herd animals, and generally find themselves gravitating towards one another with a few exceptions. Procreation requires two individuals, although modern science makes it very possible to have little contact with the other human.
Outside of the scientific and evolutionary bits, I find it wonderful to allow myself to be vulnerable enough to admit the importance of another person to me, to long for him/her, and also to know that I am self sufficient enough to be my own person. It's just nicer when I don't have to do everything alone. I've always been more of an individuated person, and found it wonderful to learn how to have full attachments with others, including admission that life is better, more rich, and wonderful when it's shared. Need? Not really, but "strong desire", "instinct", "longing", absolutely!