Have you ever been trying to run a download or load a page, and it just stops for a minute, right in the middle? Or worse, you're playing some real-time online game, like Quake, Team Fortress Classic, Unreal Trounament, or Ultima Online, and everything just stops. Or maybe Diablo 2, and the Black Wall of Lag stops you from moving. Isn't that just about as annoying as a loose hoe handle? Doesn't it make you just boiling mad? Don't you wish you could throttle the moron responsible?
Would you believe you have probably done that to other people, several times over?
One of the things that causes this kind of internet slowdown is when some internet server, already running at full capacity, gets the same email message 100 or more times over, going to everybody on your mailing list. It's really very bad manners to send out mass mailings. In fact, some of the most annoying and troublesome viruses and worms are the exact same thing -- mass messages going to everyone in your address book or contact list -- only automated.
And chances are pretty good that your chain letter is bogus anyways. The common features of bogus chain letters are pretty simple -- some sort of hook (typically the promise of good luck, or else that forwarding it will help some unfortunate), a threat, and the request to forward it. Many include an example or two of people who benefitted from forwarding the thing, and people who suffered because they did not. And while it's possible that the stories are not made up out of whole cloth, it's still not surprising that they can find examples of each.
Let's take an example of a chain letter. Suppose this chain letter promises good luck if you forward it to ten people, and bad luck if you delete it. Suppose that it starts out sent to ten people, and of them, three forward it and the rest delete it, and so it goes with every time it gets forwarded. After six generations, the message will have been sent to 2,390 people, of whom 706 have forwarded it. That's 1,684 people who have deleted the message. Do you suppose you could find an example of good luck out of 706 people, over the course of two or three days? How about an example of bad luck out of 1,684?
Let's change the numbers a bit. Suppose it gets sent out to 30 people each time, instead of ten, and five of those thirty forward it. After six generations, it has been forwarded by 16,717 people and deleted by over 100,000. Now do you think you can find examples of good and bad luck?
It's a classic example of the utterly fallacious post hoc, ergo propter hoc reasoning -- essentially saying, "because this happened after that, that caused this to happen." Forwarding chain letters is a waste of resources that could be much better used for the communications that we really want to get.
If you want to know more, go look over the government's hoax page, particularly the section on recognizing hoaxes.