I have been really bothered by stupid chain mails and forwards. Sometimes I feel harassed by these messages too. So I tried to look at the probable causes of chain mails. If you think there are some other reasons, let me know and I will append it to the list.
1. The way chain mails are designed, any mail receiver tends to send the mail to lot of other people in their addressbook. Some people are so smart that they click on their complete addressbook, and send these chain mails to everybody. What happens is that in four-five hops, the amount of mail becomes more than a million and can easily get the server of a mail company down.
AOL gets hit by it most. Because most people using AOL are smart, and capable enough to know how to send these mails to most possible users. Other companies are also getting hit by it.
2. When you send these chain mails, you not only expose your email addresses, but the email addresses of thousands of other people. Spam companies recognizes these addresses and harvest them to send more spam.Now, if you send the messages to multiple users, most servers categorize you as spam sender. So, it may happen that your genuine mail may not reach the right destination if you have been categorized as a spam sender.
So in yours and everybody's interest, don't send email which are not authentic. Don't send emails which say send it to so many people or you will die (or whatever), or delete that portion of the mail. Always Blank CC the addresses, if you want some special mail to send to your "friends and non friends".