I think that knowledge and curiosity are the keywords here. Knowledge; to know something, to ad new knowledge, to know what you know, and know what you don’t know, as well – that’s a driver for most people. It almost becomes scientific – for instance I found the following in an article (http://hypography.com/forums/articles/7546-fascination-science.html on 6th of April 2008) - quote:  

“Indeed the word science is derived from the Latin word scire, meaning, “to know.” Science is dynamic, which means it is constantly developing, the science today is not exactly the same as it was, say, fifty years ago. This is so because scientists are always on the look out for new knowledge. To gather new knowledge, scientists have to constantly ask questions, starting with What, Why, When, Where and How. Asking such questions is curiosity”.  End quote. 

I think that the above quotation frames it well. Man is curious by nature, and to learn what you know and don’t know about a subject, that, I think is a great motivator for people to take quizzes. Then there in addition is the movement towards self-assertion, the endeavor to become the smartest, which is also a key driver, the desire of showing off, lies deeply in most people as well. 

So you are well excused, when you spend time on quizzing