Friar Lawrence wants to slow down the marriage preparations for Paris and Juiet because Juliet is already married to Romeo. Romeo and Juliet need time to figure out how and when they are going to explain their marriage to their parents.
Paris says that Lord Capulet wants his daughter wed quickly because it will hopefully ease her sadness. He thinks that by marrying Juliet and Paris she will be so happy that she will stop grieving. In actuality, her father's demand that she marries Paris makes her even more upset and inconsolable.