Scenario 1: Girl (Mine) is engaged to a guy due to familial pressure. She doesn't really love him, but he makes sense on paper. Guy (yours) and her start to fall for each other. Drama! They could also be exes, which would just add a whole other layer.
Scenario 2: Girl (Mine) have been bffs forever. They kept missing the boat, but they're pining for each other and eventually some event pushes them over the edge.
Scenario 3: Girl (mine) and guy are step-siblings. Maybe their parents married when they were in their teens, but they got close and secretly want each other. Parents aren't happy.