Google introduced personalized search results as an experiment in 2013. It basically used to display relevant information from their other products like Gmail, Google+, Calendar etc in search. The feature was also not enabled by default for all and the criteria for enabling is also not made public.

If you search for keywords like my purchases, my events, my bills, my reservations, my photos etc there is a pretty good chance of google showing it to you. In fact being not evil, Google will first ask you if you would like to enable showing personalized results. And here's what you will see-

How to disable personalized search results.

To disable or turn off private results, visit Search Settings page, scroll down to"Private results" section, select"Do not use private results" and click on Save. This will permanently disable the feature until you choose to turn it back on.