I believe that what we call "love at first sight" is actually based on YEARS of subconscious analysis of behavior in people we love who are in our lives or have been in our lives. We have made mental notes, not even realizing it, of physical characteristics, speech patterns, non-verbal communication, linguistic expression, intelligence, characteristics of kindness and empathy, even religious leanings that are recognizable in a very few minutes, especially by people who are naturally intuitive. It's no surprise that our spouse has characteristics of a mother, a father, an aunt, and uncle, a friend whom we love or have loved. We associate those characteristics with the personality of the individual and recognize them in another quickly. Many times, the physical characteristics of an individual correlate to his or her personality. If you walk through a store with a baby that is exceptionally cute, watch the reaction of the people who meet this child. All positive reaction goes into shaping of the personality. But recognizing physical characteristics and relating them to personality is one aspect. I believe that the whole process entails a more complicated process. All of our observations are subtly recorded in our neural pathways and we bring that information with us to our first encounter with a new person.
So yes, I do believe in love at first sight. It can happen with friendships as well. There are some people that you know you are compatible with after only a few mintues. Again, it's all based, I believe, on subconscious recording and classifying of complicated observations and analysis that we make on a daily basis.