what is the dramatic significance of Lorenzo and jessica love story??

There are a few ideas at play here. Jessica is obviously a daughter who is rebellious against her father's strict orthodox ways. She uses Lorenzo, in part, to get back at her father. Marrying a Christian man would most certainly cause Shylock endless grief. I might question Lorenzo's love for Jessica but certainly having a Christian wed a Jew adds a level of irony to the play considering the sharp divide between Christians and Jews in the play.