Watchlist entries do not have history of how they were added or why. Some people put effort into identifying a reason for each of their watchlist entry, and in some cases do not see why it was added. I'd suggest to log watchlist changes: when and why a page was added to watchlist. For example

April 3, 2011 Foo was added to your watchlist because you created the page

April 12, 2011 Bar was added to your watchlist because you edited the page

April 14, 2011 Baz was added to your watchlist because you added it to watchlist yourself