I think most people agree that slightly more 'realistic' declarations of war would be nice, but after several years you already own more provinces and are more powerful than any other nation. So under the proposed solution of 'they shouldn't declare war if they're small and just signing their own death warrant', no country would ever be in a position to declare war on the player.

The game is total war, not a simulation... I just feel people take it a bit too seriously when the game does something a little stupid. I'm sure someone will mod the diplomacy to suit you in the future, but for now just live with it - and if you really need to justify the declaration of war, just assume that one of your princes / ministers had an affair with the friend/enemy leader's wife, and they didn't take too kindly to it. Problem solved!