Attraction is often thought of as a magical thing. Two people will meet and feel immediately attracted to each other, whereas another two, though they like each other personally and acknowledge each other's good qualities, will feel only friendship. The latter case is often explained with phrases like "they just didn't feel it" or they "had no spark", whereas the former, if it were to lead to marriage, might be deemed "love at first sight." The couples themselves might not be able to explain pre...