Many mental health models attribute defensiveness or reactivity to a character flaw in the client. Motivational Interviewing takes a different perspective by viewing defensiveness as a natural byproduct of certain kinds of interactions initiated by the counselor.
To translate that to real life relationships: If the person I am talking to becomes defensive, it's usually a signal that I overstepped my boundaries in some way, and I need to back off. Most of the time their reaction does not mean they are touchy or too sensitive or can't handle the truth. There are exceptions, of course, and some people are much more tuned in to this type of violation than others.
Most often, defensiveness arises because we have been disrespectful by implying that we know what someone else should do. This can be very subtle - it may feel to us like we are just being helpful or offering a solution the other party may not have considered. We may even feel really good about being able to offer our insight in hopes that it will solve their problem. Nevertheless, if we dispense even well intentioned suggestions or advice without first asking if the other person is interested in our opinion, we risk triggering resistance.
Implications that the other party screwed up, is about to screw up, or is otherwise less than capable are other common defensiveness triggers. Interrupting, talking over someone, telling them they are in denial or not seeing clearly, or jumping to inaccurate conclusions are also likely to set it off. Most people simply do not like being told what to do, or that they are wrong.
Of course, being abundantly human, we will all trip into this territory at times. So it can be helpful to know how to back up, repair the damage to the relationship, and restore harmony. Here's one way:
If I notice a friend becoming defensive, I can pause, take a breath, and look into his or her eyes. I can say, "Uh oh, I think I might have stepped out of line there. I'm sorry -- does it feel like I am telling you what to do or saying you are wrong?"
Then I can listen to the response, and reflect it: "So when I suggested craigslist, it sounded like I thought you needed my help finding work. Of course you can handle your own job search! I'm sorry about that. Please go on. I will focus more on my listening."
What's amazing is that even people we thought were impossible to deal with usually respond very well to this approach. Most folks really appreciate having their autonomy respected, and will readily forgive us when we take responsibility for our transgressions. This dance is not always an easy one, but it seems worth it to me...