Last 10 Millimeters

While riding home on the bus one day, a girl sits down by Sai and lays her head on her shoulder while she sleeps. When she leaves she leaves behind her lip balm. Sai then becomes obsessed with returning the lip balm, taking the same bus every day. What will Sai do when she finally meets with the lip balm girl again? What are the feelings that underlie her obsession?