Defensive driving encompasses many different ideas and practices, one of which is understanding the effects your driving may have on others, and the results this can have on your safety and theirs. In my state of Arizona, there is a law that if another driver approaching from your rear signals by lights or horn their intention to pass you, you must allow them to pass, giving way to the right if necessary to let them around. Many other states have similar laws.
How many times, though, have you had this attitude? "I'm going the speed limit, that guy can just slow down, he doesn't NEED to pass me." I suspect there are few of us that haven't had those feelings at one time or another -- it's just human nature; but it's human nature that we need to overcome, for a couple of reasons.
First, you don't know the reason someone wants to pass you, and while it may be an unusual case, who would want to be in the way of someone who legitimately needs to get somewhere -- like a hospital -- quickly. It doesn't matter that in most such situations, it is better by far to call for emergency help and have the person transported by professionals, in an official emergency vehicle, for many good reasons. In some cases, especially in remote areas, there is no choice but to transport a sick or injured person yourself. On the outside chance this is the case, you should never prevent someone from passing.
Second, each of us, in my opinion, should drive so that the drivers around us stay calm and in control (as much as that is possible), for our OWN safety. You do not want someone behind you that is pressured, frustrated or angry. Preventing another driver from passing you creates unnecessary anger and frustration. In an unsafe attempt to get around you, your obstructive action and their anger can combine to cause serious mishaps, maybe even resulting in injuries or death to you, to them, or to some uninvolved third party. In such a case, you would be partially responsible for the problem, if not legally, certainly morally.
If another driver wants to pass, make every effort to HELP them do it, even pulling off the road if necessary. Doing so takes mere seconds and it is a courteous, safe and intelligent thing to do.
Keep it between the fence posts!