Ideas look like branches. All ideas stem from other ideas. If you are trying to figure out what someone thinks, move a little further down the tree. You'll learn a lot.
For example, let's say my wife makes the claim that a particular salad is fantastic, I respond with the claim that it is, in fact, not. At this moment, we realize we disagree. We then begin to argue.
Here are her reasons:
1. The lettuce is crisp and fresh.
2. The croutons are crunchy and have a lot of flavor.
3. The dressing is rich and creamy, better than any she has had before. This, therefore, means the salad is tasty.
I am not convinced.
The problem is she could argue those points all day and I would still not be convinced. Not because those aren't good reasons, but because the reasons don't line up with a more basic belief that I hold.
Her appeal to the freshness of the lettuce is great. That's a valid criteria for lettuce. The problem is that I don't like lettuce at all. I don't like the flavor of any lettuce, no matter how crisp and fresh it is. So the argument will never convince me.
Her argument for the goodness of the salad can only work if I first get to the point where I accept the goodness of lettuce. Until then, arguing about the freshness of the lettuce is completely useless.
Let's look at a more political example — abortion.
The topic of abortion is an important one. And the debate is charged and practically endless. (Do note that this example is a little different than the salad. Abortion not an issue of preference, but of morality — of truth.)
One of the major problems with the debate on abortion is that we don't really understand why people think what they think. When the topic comes up, it's typically a fruitless back and forth about women's rights. The question most often asked and argued is, "Does a woman have the right to end a pregnancy?"
Now, this is a good question, but it doesn't get to the heart of the question for most pro-life people. For pro-lifers, the more basic, and important question is, "Is the unborn a human being?"
The pro-life person is not against women's rights. They just think that nobody has the right to kill a human being without proper justification. And the unborn is a human being.
And on the other side, the pro-choice advocate is not in support of killing babies, they just don't believe the unborn are real human beings. So for them, the question then becomes, "Does a woman have the right to end the growth of something inside their body?"
So if you are pro-choice, you'll never convince your pro-life friend of your view by talking about a woman's right to control her body. For the pro-lifer, the life of a human being is almost always more important.
Disagreement on any topic often stems from a disagreement much lower in the branches of belief. When you disagree, find the source.
This is also why we are often baffled by a persons beliefs. If your assumption is that the unborn are human, then someone supporting abortion seems illogical at best, and immensely immoral at worst. But if you understand that there is something further down the belief branch that is probably causing the disagreement, you'll realize they're not crazy or immoral, they just believe differently.
The pro-choice advocate is not a baby killer.
The pro-lifer is not against women's rights.
The theistic evolutionist is not anti-Bible.
The creationist is not anti-science.
The arminian is not ignoring the text.
The calvinist isn't uncaring.
If you "have no idea why" a person would think what they think, you probably aren't listening.
You probably aren't asking enough questions. You're probably more interested in proving them wrong than understanding their position. Rather than moving down the branches, you're probably assuming you are starting at the same branch and they are making a leap somewhere else, into irrationality.
If they seem crazy for not liking the salad, make sure they actually like lettuce. Your arguments will be a lot more fruitful. Only when you get to the root of the problem will you be able to have a meaningful conversation.