We meet people everyday. Strangers who might be passing by in a shopping mall, the driver in the car next to yours at the red lights, a greeting with a friend's friend. Many will simply slip by without notice. When Chance decides to stop and allow an encounter, with some, there is an automatic click, where one goes "ah, we can get along". Perhaps this is the all-elusive "spark" that some are looking for. Others, we are more cautious in approaching or getting close, though that can certainly change with time. Yet others, we will instinctively avoid or even have a mild dislike right from the start.
While I do not personally believe in phenomenons such as love at first sight (how do you even know it's love when you don't know anything about a person? Can one really understand their own feelings within a mere few seconds of first sighting? I think not..), our power of observation and intuition is something we rely on more heavily than we know. The way we've been brought up, our prejudice, our bias, our preferences are all attributes to how we deal with the situation where we meet someone for the first time. Feelings are not irrational, they can be explained- if one allows it to be explained, but often, Denial pays a visit and Logic is cast out.
We all start off as strangers at one point, and perhaps after a certain period of time of getting to know someone and becoming friends/partners, we become strangers once more, almost like an expiry date stamped onto a relationship.