I imagine many here have experienced a period in their lives when they were acutely aware of something being 'missing', something intangible, something that's hard to define. I was lonely as a child, I had a small number of friends, sure, but I couldn't shake the feeling that I didn't fully connect with them (I was never anybody's 'best' friend, it just didn't happen). I always felt slightly off to one side, always wanting something 'more'. I don't know, maybe that makes it natural for me to empathise with both Oskar and Eli (and even Håkan) in this wonderful story. I don't want to be presumptuous and speak for others but those are my thoughts about your question, a taste of loneliness. Of course, there will be those that didn't have the same feelings as mine, and they will have their own reasons and causes for becoming infected.
Oh, and if you think your post was too long, you mustn't have come across any of Sauvin's posts
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. At first I found it hard to believe that I liked a love story because of the actual romance (I had never watched a film because of the romantic side except titanic). But when I think about it, everything seems to be the cause of our infection. So I agree that while I may never find out exactly what it is that draws us it doesn't matter anymore, it happened and I like it. 