You can’t answer this question without a little speculation, because we’re talking about human motivation/psychology. Leaving aside the small percentage of clicks that can be attributed to click fraud and accidental clicking, here are some of the reasons that Google users might click on an AdWords ad:

  • They don’t know it’s an ad. See above. Some users can’t tell the difference between paid and organic results, and they wouldn’t be doing a web search in the first place if they didn’t intend to click on something.
  • The ad is the most relevant result for their query. If the searcher is looking for a specific product or service, with clear intent to buy, an ad could well be the best “answer” to their “question.” Product listing ads, which include a picture of the product in question, can be especially appealing, because they show you exactly what you’re looking for. Google’s Quality Score system works to make AdWords beneficial for everyone – if Google serves up only the best, most relevant ads, they are more likely to be clicked, which is a win-win-win situation for advertisers, searchers, and Google alike.
  • The ad catches their attention indirectly. Sometimes an ad is served that is only indirectly related to the searcher’s query. Or they may not be searching at all – they may see an ad on the content network (while they are checking their email or reading a news article, for example). Nonetheless, the ad might be relevant to their interests.

Any of these reasons could work for your benefit, but the safest path to success as a PPC advertiser is to try to make your ad the most relevant result for the search query. Then they’ll have no reason not to click. (from wordstream)