An honest answer to this question is ...there are many reasons.

The Boy Who Cried Wolf ...tells a story of how the adults were led to believe there was danger when there wasn't, then they believed there was no danger when there was.

Why are people led to believe things??

How about Adam & Eve??

Eve was not only persuaded to consider the thing which she believed to be true, was in fact, not true (though it really was); but she was also led to believe some other important truth was being withheld from her ...and we can see how she viewed that as a very unloving motive.  And that is precisely the way it was intended for her to perceived it.

And whether one feels tricked, duped, or deceived ...those are very strong feelings that often emote rather immediate responses.  Yet, how much more intense are those emotions when she finds out the friendly advice was beyond mere trickery. 

Not an acquaintance, not even much of an introduction ...can't really be called treachery or betrayal, as no trust had even been established.

So tragic how a simple idea, before it can even entrench itself, can quickly spark doubt and mistrust. 

A very important point to be made ...is that we must realize, that particular event was not a one-time strategy.