Why consumers open E-mails

I found this article interesting because I have seen many articles writing how we make consumers open E-mails, but not why consumers open E-mails.

http://www.wilsonweb.com/email/wilson-why-open.htm

Discussions so far has been a kind of "product out" thinking rather than "customer in" way of thinking. This analysis is just an first step to understand why consumers open e-mails, but this type of further analysis would lead to know consumers much better.