Sometimes it can be really hard to get players to concentrate on a particular goal for an adventure. They'll do anything they can think of to avoid actually making any progress towards what they're supposed to be doing.
Other times, the players get some idea stuck in their head and absolutely will not be diverted from it, no matter how important it is for them to stop being so single-minded for a minute and pay attention to something else.
Interestingly, both of these can be infuriating for a GM.
Obi-Wan: We head off to Coruscant.
Sio Bibble: But our planet is blockaded! Look! Our ships and pilots have been captured!
Amidala: You must help Us in Our fight for justice on Naboo. Jedi knights would do no less.
Obi-Wan: But no Jedi knight would turn away from a prior quest already accepted. We must recover the Orb.
Sio Bibble: But you must stay here. The Queen needs protection.
Qui-Gon: She can come with us.
Panaka: Then we'll accompany you to Coruscant as well.
Qui-Gon: You're not joining our party if you're going to ninja our loot all the time.
Panaka: I will not abandon my charge to guard the Queen.
Qui-Gon: Fine. But we're not taking Bubble. That goatee is a death sentence.
Obi-Wan: Uh... don't you have a—
Obi-Wan: Can I see your character sheet for a—
Qui-Gon: You may not.