Tim Spalding, The inventor and CEO of LibraryThing - a social booksharing site, have some got points about reasons why people are tagging. In an article from his blog Thingology, he makes a statistical comparison between Amazon and LibraryThing tags and one of his conclusion is "Tagging works well when people tag "their" stuff, but it fails when they're asked to do it to "someone else's" stuff.".
See: www.librarything.com/thingology/2007/02/when-tags-works-and-when-they-dont.php
