I am of those people who hates to shop. When I do I like to get in and out of the shops as fast as possible. This normally means I am totally focus on I went in to get that I forget to get the other things that I may need.
For instance on the weekend I brought a new iPhone. I get home and the first thing I realise is that I need a protective case for it.
During shopping at the store the sales attendent tried to sell me Mobile Me and Apple care which I repeatedly said no to. This caused me to have bigger blinkers on as he tried to push harder to sell me these two services instead of something I wanted.
He had the prefect opportunity to sell a case to me. I imagine nearly everyone with an iPhone or iPod buys a case as everyone I know has a case for it.
So instead of doing the amazon thing and suggesting to me a product that would useful to me based on others purchases he miss out selling me something I wanted and made me annoyed.
The lesson is to look at customers as people needing suggestions of stuff they would like to buy not people just to buy what you immediately want to sell. Amazons model of people who purchased this also purchased is a great to identify those products you should be cross selling.