Mielr said:
What explanation is Han given for Luke's departure from Endor?
"Oh, Luke, yeah- he, um, went out for a gallon of milk- on the Death Star. It was more important than staying here and helping us with the shield generator"
Luke felt Vader's presence on Endor and felt he was endangering the mission. So he decided to confront his father... but not before telling Leia she is his sister.