Romeo and Juliet met at the Capulet's party and kissed. Romeo and Juliet both lived in the same city which deals with proximity. They both were geographically near each other which increases attraction and gave them the opportunity to be around each other. They had families that despised each other and often fought so their names were probably familiar to each other. Mere exposure effect says attraction is increased with things we are more familiar with so their attraction to each other was probably influenced by their familiarity with each other.
Tybalt sees Romeo at the party and challenges him to a fight because the Montagues and the Capulets do not get along and shouldn't intermingle. Their families have a lot of similarities so Romeo and Juliet grew up in similar family dynamics. Both of their families were rich and lived lavish lifestyles. They were also both the only child. They had a lot of similarities in lifestyle and circumstances and similarity increases attraction.
Romeo comes to Juliet's balcony where they exchange vows to get married the day after they meet. Since it is so early in their relationship they are experiencing passionate love. Passionate love is characterized by intense feelings for another, thinking of them often, and wanting to be near them. Romeo shows this by often surrounding himself with Juliet and Juliet shows this by constantly thinking about Romeo. They are not at companionate love yet which comes later in a relationship and involves intimacy and intimacy takes time to form through equity and self-disclosure which they haven't really had the time to practice yet.