I assume you are speaking of Romeo and Juliet. In that case consider how many people believe in love at first sight. Also consider that they are both supposed to be attractive people to begin with. Also they are young and nearly everyone has either experienced or observed the intense feelings that often arise in the young who mistake that intensity for sincerity and affection or desire is often then mistaken for love. The point being we all know that in that state it seems like you cannot live without the other person and it is clear that the two young lovers are in that stage of the relationship