When Amazon recommends a product, it is clearly not a coincidence. What their "collaborative filtering" recommendation system does is to retarget a visitor with what he or she has bought in the past, which items they have put into their shopping cart, and what other similar customers have recommended, viewed or purchased.
J. Peng, D. Zeng, H. Zhao, и F. Wang. Proceedings of the 19th ACM international conference on Information and knowledge management, стр. 809--818. New York, NY, USA, ACM, (2010)