There are many reasons for this, some of them being the lack of familiarity of the issue at hand, and also simply downright sinfulness of the other person who is unwilling to be consistent in facing the truth, or even it is that the other person will be consistent and not see the utter ridiculousness of their consistency (reductio ad absurdum). The two latter points have happened to me many times. It can get very frustrating very fast, when you are confronted with someone who is absolutely unwilling to acknowledge the bald contradictions and the ridiculously fallacious reasoning that he holds on to, even when you directly expose him of it.
For instance, I was once talking to someone who was unwilling to acknowledge that it should be biblical standards that should govern all interactions, even "political." He then proceeded to say that it should be based on a society's "common sense", since everyone obviously understands what is right and wrong. And that it should this "common sense" that dictates what is right and wrong in society and this is the basis of our laws. (Curious that total depravity's distortion of "common sense" never entered into his thinking.)
I then asked him whether or not it was OK for Sharia law to allow fathers to murder their daugthers in honor killings because that muslim society's "common sense" says that it is OK.. He said that if this was the societal understanding, then yes, it should be OK. Really? Are you kidding me?
I then told him that thinking this way makes him logically inconsistent, in saying that that honor killings would be OK in muslim societies but not in the US. How he can acknowledge that murder is wrong, yet think that honor killings are OK? Of course, he didnt see how he was being inconsistent. He also is a Christian (with a very weak worldview).. Needless to say, this was a very frustrating conversation because he was absolutely unwilling to see how logically inconsistent he was being.
This further serves to remind me that being logical really is an ethical and moral imperative. If you are unwilling to acknowledge that honor killings is murder, yet murder as you understand it in our country is wrong, then that inconsistency isn't simply an intellectual issue, it is a moral and ethical one. And the implications and the applications behind this thinking is certainly chilling and can have a devastating effect on our society.
This brings me to my final point in that no matter how frustrating the conversation can be, I need to always remind myself that it is absolutely critical to try to present your arguments in a way that is familiar and understandable to the other person. Perhaps with the situation I described, I could have been better at that. Notice that I didnt say *relevant.* When talking about ultimate truth issues and the Bible etc..., whether your conversation partner admits it or not, truth and the Bible always have bearing on a person. It is always relevant to that person. God's law and the pronouncement of sin is always applicable to a person, whether he believes in God or not.
In terms of being familiar and understandable: If you try to understand and address the presuppositions that a person has, which lead to his applications in thinking, then you will be much more effective in helping the other person understand. If you simply address the applications in his thinking and never address the presuppositions, you will never get anywhere because his foundation is still secure because you never addressed them.
However, be careful not to think that it is your own power and cleverness in addressing a person's presuppositions will help the person understand your point. It is always God working through your efforts who will effect the change (Phil 2:13). This doesn't excuse us from arguing properly. In fact, if we are to understand Scripture and its imperatives, it should force us to understand that we need to in fact argue properly. If we understand that we need to be faithfully obedient in all we do (and God using our meager efforts), we must use proper logic and argue correctly.
Here is a short video on how not to present the gospel.
This is from the 1970's movie "Hardcore." This is a story about a Dutch Reformed Christian father who travels to California to search for his daughter who ran away to join the porn industry. The father somehow enlists the help of a prostitute, and they are waiting at the Las Vegas airport for some reason that I cant remember. The prostitute asks the father about the Christian faith, and the father ends up discussing TULIP. You can see that the prostitute has absolutely no idea what he is talking about and end up moving on to something else.
This is a perfect example of how you can preach the truth (and everything that the father said about TULIP is exactly correct), but still not be able to reach your audience at all because you never took the time to understand the other person's presuppositions and address those things first, attempting to help the other person understand and also become familiar with what you are attempting to inform him about. (Also, I almost forgot: at around 2:30, there is some crass language.)