Why did the giant space worm try to swallow the Millennium Falcon in Han Solo 12:17?

Do giant space worms just live in asteroid fields waiting for unlucky space ships to fly into tunnels to escape the evil empires patrol ships but the tunnels happen to be the mouth of the giant worm? Seems a strange way to make a living to me, I mean, how often do space ships fly into asteroid fields to escape the evil empires patrol ships? But I can see the important moral lesson in this story and that is; it is far better to face ones problems head-on rather than to go hiding in tunnels....because those tunnels could turn out to be the mouths of hungry giant space worms.