When Luke is leaving Dagobah to save Han and Leia, Obi Wan says:
If you choose to face Vader, you will do it alone. I cannot interfere.
Why? As a Force-ghost, Obi Wan can presumably go anywhere he chooses. Is he being arbitrary and stubborn here, or is there some reason that this time, unlike many other scenarios (Death Star I, Yavin, Hoth, Dagobah, etc), he can't accompany Luke? In essence, is the issue that he can't help Luke, or merely that he won't help him?