Causes vs justifications of morality

Have I ever mentioned that I am in SANE, the UC Berkeley secular student group?  Anyways, in an earlier meeting there was some sort of moderator fail, and we spontaneously started talk about all the most trite atheist topics imaginable.  Case in point: "Where do your morals come from?"

Among the many answers, there were the usual ones claiming that we evolved morality.  I think this is a fairly reasonable claim (though possibly incorrect), and there are several possible mechanisms for the evolution of morality.  However, I am far from an expert on the subject, and would only offer a link to Wikipedia.

Instead, I would like to talk about why I think this answer misses the point.

It's easy to miss the point of the question, because its wording is ambiguous.  If I ask where something comes from, it's perfectly reasonable to think that I am asking what causes it.  But I think in this case the point of the question is to ask for a justification for morals.  Not, "What causes you to have morality?" but "Why should you have morality?"

I believe the question gets misinterpreted because its "correct" interpretation is kind of senseless.  When you ask why you should do something, you are asking for a moral or ethical justification.  Thus the question is asking for a moral justification for morals.  It's asking for something circular.  I don't think anyone, religious or otherwise, could possibly answer the question in a satisfactory way.

In my mind I'm imagining several fictitious conversations...

"Why should you be moral?"
"God wants us to be moral."
"Why should you do what God wants?"
"Because God created us."
"Why should you do what your creator wants you to?"
"Because it is good to honor one's creator."
"Why is it good?"
"You can't just keep asking why!  At some point you'll hit the bottom!"

(My mind's Christian is probably not very accurate, by the way.)

"Why should you be moral?"
"To go to heaven to be with God."
"Why should you try to be with God?"
"Because God will give us eternal happiness."
"Why should you try to achieve that?"
"Wouldn't you like to be happy forever?"
"Yes, but why should I try to get what I like?"
"This is pointless!  You can ask why about anything!"

I imagine many atheist answers failing for similar reasons.

"Where do your morals come from?"
"We evolved a sense of morality."
"Yeah, but why should you try to get what evolution wants?"
"It's not what evolution wants.  It's what evolution caused me to want."
"But why should you try to get what you want?"
"I misspoke: it's not what I individually want, but what we collectively want."
"There's still the same problem.  Why should you try to get what people collectively want?"
"Would you argue that something is good even though it goes against what everyone wants?"
"That's not a justification."

The evolution answer misses the point, because all straightforward answers miss the point.

And lest this post be all about tearing things down, let me suggest a better question, a pragmatic one.  If I think someone else is doing something evil, how can I convince them that it is evil?  To use persuasive moral reasoning, we must have some sort of common basis, even if it is not a fundamental basis.  So what is that basis?