Just about everyone acts in their own self-interest. There are degrees to this. There's the sociopath who'll happily kill you to get five dollars for a cup of coffee. (And who, I suppose, enjoys the killing.) There's the person who gives five hundred dollars to a homeless shelter because he thinks it's the right thing to do. (And presumably receives some psychic rewards from having done so.)
Sometimes the self-interest is short-term. For a married woman in some countries, it might be "I do this so that my husband will not beat me." In the long-term, it might be "I do this so that I will go to heaven."
And enlightened self-interest is even more complex. "I pay taxes to support the society that I live in." Of course, that's bundled with "I pay taxes so I don't go to jail", so maybe I should pick a different example. How about "I vote to support the society that I live in"?
Ultimately, you do things to benefit yourself and the people you care about. That might be your family, your friends, your neighbors, your social group, your town, your country, or your planet, depending on what motivates you.
So what does this mean when we try to understand the motivations of others?
Start with this: we all want to be the hero of our own story. Maybe that's not always true, but it's way up there. And we like to think that we understand our own motivations and that we act on them in the way that we do because we are truly trying to accomplish good.
Here's the sad thing: that applies to Osama bin Laden too. And to the nut who flew a light plane into an office building in Austin recently. They are the heroes of their own story.
Most of us beg to differ.
Mercifully, in the United States at least, beliefs like that are outliers. The mainstream of American thought may be broad (if not always deep), but it doesn't tend to go there.
When we're trying to persuade other people, we need to first understand what motivates them. Without that, you've got no lever and no place to stand.
It's actually worse than that. By not understanding what motivates others, you can actively damage your chances of recruiting them to your position. What's obvious to you isn't obvious to them, because they've got a different set of postulates that they've developed from their own life experience.
That doesn't make them stupid.
You need to go after the postulates, but you can't as long as you don't acknowledge that they exist or while you're busily targeting the wrong postulates because you don't understand the person that you're arguing with.
And you'll just end up yelling at each other.
Which benefits no one.
Sometimes, there will be absolutely nothing that you can do to reach consensus, even if you do understand where the other person is coming from, because there will be postulates that they (and you) have that cannot be changed and cannot be modified and that are in ultimate conflict.
Not always though.
And I'd rather try something that might work than something that's guaranteed to fail.