A lot of people want an argument, or like to setting up a platform so that they can air their views on a subject they feel strongly about.
If you choose a topic which no one really cares much for, or does not have extreme points of view, you'll hardly get any posts or comments.
I.e If you create a post saying "I'm giving up smoking" you would get a few "me too" comments, some "well done keep it up's" but that would be it.
Starting off a post announcing your point of view on a controversial subject, or subtly criticising a certain political view, sexual practice or emotive issue will have a fair few people taking the bait.
Many people enjoy the opportunity to have a debate about an issue, or use it as a soapbox to announce their views to a larger audience. Some are confused or are interested to find out what others think.
However for a lot of hubbers I expect that they do it for a bit of fun, or to just interact with other hubbers.