Matthew Chapter 8

Mat 8:34 And, behold, the whole city came out to meet Jesus: and when they saw him, they besought him that he would depart out of their coasts.

Now after we read this verse we can ask: Why do the people want Jesus to leave? Isn't it something great when to demon posessed people get healed?
To answer this question we have to imagine how and under which circumstances the people experienced this -actually great- wonder of Jesus:
They feared Jesus because of his power and might (Jesus was mighter than two demon posessed men, who were so dangerous, that no one could go the street they were on V28).
I also think, that those two, who kept the pigs, were so shocked and frightened, that they exaggerated the whole story. And as you all know, message which is spread by people will change in the end (example: person A tells person B:" I like you". Then person B tells person C:" Hey, person A loves me!". Afterwards person C tells person D:"Did you know that person A and B might marry soon?"- from like to marry!! quite a difference!).
So the story of Jesus casting out the demons was so changed, that fear was more than joy. They couldn't see the love and mercy of Jesus behind that man made "curtain". Those are obstacles made by mankind.

I think it's really interesting to understand how and under which circumstances the people in the Bible experienced something. Sometimes we read through a story without knowing what consequences that story had for the people at that time.