"And when he had called all the people unto him, he said unto them, Hearken unto me every one of you, and understand:
There is nothing from without a man, that entering into him can defile him: but the things which come out of him, those are they that defile the man.
If any man have ears to hear, let him hear."
I like how Christ taught when he was on the earth. He often gave people something to think about, and let them discover the lesson themselves, as they figured out the principle. His disciples asked him about this one, and he explains it further later in the chapter. But I don't think that it is ever just the lesson that Christ was trying to get across. He wanted... and wants... people to learn how the world works beyond just this individual lesson. And of course to think about it, to gain a testimony of the principle, and to put it into action in their lives. Perhaps in this case he wants us to think about how a person becomes defiled... how sin starts, and to think about all the external things that we try to to blame our problems on. :) If he just said it straight out, like "take some personal responsibility for your lives" we might think less about it, and perhaps it would be easier to blow it off as a general principle when we haven't really thought about how to apply it to our lives.
This method of teaching, because it requires thought and introspection, seems really effective. When we realize the lesson on our own, our minds and hearts are both engaged, and we're not in defensive mode. That's part of what reading the scriptures does for us as well. We don't read just to have the words flow by our eyes, but so that we can think about it, and ponder how it applies to our lives... and do something about it.
Today, let us have ears to hear, and let's think about the lessons that God sets before us, including this one, and how they apply to our lives now. And then let's take some action in our lives to improve.