My students want rules that give answers. When I insist that they've got to instead give me arguments about why their positions are right, they reply, "But that's just my opinion." (And I can often here an interrogative rise at the end of that sentence.) I reply, "It is your opinion, but not just your opinion. Do you think your 'opinion' is the better one? If you convince me why, you'll have not just expressed an opinion. You'll have persuaded me that your position is the better one."
Taylor Mali says it all better: