I went to my usual repair shop to ask for a general check-up on my 2000 Odyssey before driving out of state to visit friends over a period of a week. They replaced the valve cover gasket (at $436). When filing the paperwork I noticed they had done the same thing almost exactly one year ago. Four days earlier and it would have been under their warranty. I don't remember ever having to replace this gasket before last year!

Any ideas on why the gasket would fail after just one year?

(Note: I had a tune-up done six months ago.)