Why does Mikage decide she wants to move out? Why doesn't she?

Mikage decides that she wants to move out because she hears that Yuichi is slapped by his girlfriend. His girlfriend doesn't approve of the idea of Mikage living with Yuichi and Eriko. Mikage does not move out because she feels so accepted by Eriko. It is Eriko and the items she brings home that stop Mikage from moving out.