That is a very good Question you ask there . it really depend and vary from user to user . 1.An user might click on the ads cos your website is poor and lousy , and the ads happen to be the gateway for him or her to get out . 2.with the relative info you have on your page you have direct the mindset of your reader to click on the ads happening to be flashing on the side and the mouse happen to be near it . 3. just one thing many people miss out is the Search bar , from Adsense . you do get money if someone use it and click on the ads . if you need more serious advice . feel free to contact me
You can use the keyword tools in google adwords menu to find a niche that many people search and has a high cpc. When I see it might be insurance, make money online, loan, credit card etc