You pointed out that Federer was a great player, presumably as a reason why you want him to win Wimbledon. I didn't think the fact he is very good at tennis should be a good enough reason for this, hence my question. Maybe you like his style of play? His personality? His back-hand? Whatever. Backing the best in every competition - simply because they're the best - would get kind of boring IMHO.
As for backing the under-dogs, that might just be because I'm Irish. We're always massive under-dogs in any competition we enter so it's my natural inclination to not want to support favourites. I support United for different reasons though. Mainly because my old man supported them but also because they always played exciting, attacking football (even in the relatively dark days of the 80s, when I first got bitten by the bug)