Biblical support for why something is a sin? I have heard, seen, and been in religious debates in the past where one of the participants eventually falls back on a line something akin to "it's a sin because the Bible says not to do it" or "it's a sin because God said so". Understandably, this sounds and feels like a cop-out to many people, especially people who have not acknowledged the sovereignty and authority of God. It is not surprising that they want to know the reasoning, to understand the why and not just the what of an imposition.

I am far from an expert on the Bible, having only read it in its entirety the once. However, it is my instinctual estimation that most, if not all, of the sins (and requirements) that the Bible enumerates are given at least some degree of justification, either inline with the admonition or elsewhere in scripture. 

I know, however, that this is just an instinct, which is why I'm bringing the topic up here, in hopes of finding either support or reasoned contradiction of the theory. This leaves a few questions:


Can you cite any examples of commands, admonitions, prohibitions, etc. in the Bible where there seems to be no explicit justification?
Is there any scripture that talks about whether God explains His commands or not? For (slightly comical) example, does God ever say "I will not give you commands you do not understand" or conversely "I'm the boss, deal with it"?
If there is a command without scriptural justification, what is the right course of action in defending the command? (I realize this last question is more about apologetics, but I am curious about this, too)
It's like when your kid asks questions.

Kid: "Why do you work?"
Me: "To earn money."
Kid: "Why do you want to earn money?"
Me: "So that I can support you and mommy."
Kid: "Why do you want to support me and mommy?"
Me: "Because I love you and mommy."
Kid: "Why do you love me and mommy?"
Me: "I love you and mommy because Jesus told me to love you and mommy."
Kid: "Why did Jesus tell you to do that?"
Me: "Because he loved us first."
Kid: "Why does he love you?"
Me: "Ummm....."


Ultimately, every question we ask about anything in scripture will ultimately come down to an "Ummm..." at the end of it.  In other words, at some point, we have to accept that some things are true and good, simply because they're true and good, and are based in the character of who God is.  Why he is who he is will always be a matter of "Ummm...". 

The issue comes, of course, when a person's own instincts about right and wrong don't agree with the scriptural, Biblical concept of right and wrong.  Of course, this comes down to the person's understanding of who God is.  Our own depravity is our handicap when it comes to determining right from wrong, and also our handicap when it comes to having the humility under God to admit that we might be wrong.  Combine that with our educated society, and you have a culture of people who won't believe anything they can't understand.  

Mark Driscoll puts it this way: Our education has surpassed our intelligence.  

The problem with your first question, is that any explicit justification that would be given anyways would ultimately come back to the fact that God gave us the command, which would ultimately be based in his character.  Love is defined by God (because God is love), not by us, and the great problem in dealing with explanations of sin, is that these conversations usually have two people who start with different presuppositions.  Given that, we can't properly defend a command, without both parties understanding the nature of God.  Without God as anchor, we're all lost at sea.  

So what's the answer, if there's a command without scriptural justification?  Don't try to defend the command, try to change the presupposition.  

The only way to change presuppositions is to point people towards Jesus.  Those who don't believe in Jesus have nothing to gain from "cleaning up their act".  They're still fallen, and without Christ.  The best they could hope for is a nice, sanitized, relatively nonviolent life that would still end with their soul in hell.  Pointing the unbeliever to Jesus, and explaining his unconditional love, and his death on the cross is the best way to bring about a realization of what is right and what is wrong in that person's life.  Just as a child doesn't want to disappoint a good father, one who is in Christ will ultimately grow to grieve disappointing his heavenly father.  

In short: 


Every commandment ultimately, when put up to a depraved mind demanding answers about why, will ultimately fail when presented with any justification regarding any sin.  It's only when we stop demanding justifications that we begin to accept the command as right.  Some people stop when the justification is "so that it doesn't offend someone else", and some people stop when the justification is "so that it doesn't offend God."  
I don't know of any scripture that talks about whether God explains his commands or not.  Some he does, some he doesn't.  Sometimes, the best explanation is "For I am your God."
The right course of action is to not defend the command, but to present Christ, and allow the introduction of the Holy Spirit into a person's life defend the command for you.  Those who do not have the Holy Spirit don't need to obey a command, they need Christ.  If the person's refusal to acknowledge the command as right is preventing the person from admitting their own need for Christ, switch to a different command, then use the fact that it is broken to point them again to Christ.  On the other hand, for those who are believers, you should hopefully be able to point towards the Bible, and hopefully that will be enough. 


On a practical note, I once knew a guy who believed that some of the Bible was true, and some of it wasn't.  I told him to get a magic marker, find 2 Timothy 3:16, and mark it out, then proceed through reading the Bible, marking out all the parts he disagreed with.  

He gave up pretty quickly.  The Bible is meant to be accepted as a whole, and not in part.