Obviously, I, being British like Crab, have a fondness for rooting for the underdog. I also have my favourites as well, namely Murray and Nadal. It's been something of a quirk of mine that often I will root against a dominant champion in times where they clean up and wipe the entire competition off the board, but come round to them when they're no longer the best and have to show a different side to their character. I never liked Sampras much when he was winning everything, but I always thought there was something utterly compelling about watching a great champion fading away trying to recapture former glories. I loved watching him play in later years, when Hewitt and Safin could take him apart when he was off his game. The 2001 US Open final is a great bittersweet moment in tennis for me.
For that matter, I've only begun to really like Federer recently. His quest to add one more slam to his tally in face of younger, superior competition is a compelling narrative. The problem with Djokovic, now that Nadal is all but finished, is that he doesn't have a rival that can help define him. He's too good and in a strange way, he's being punished for being so utterly peerless that nobody can consistently give him a match. The Federer/Nadal rivalry softened Federer to a lot of people who probably got sick of him winning, it also became a great story that helped sell tennis to casual fans. I think people enjoyed Djokovic more when himself and Nadal were taking slams off each other, to be perfectly honest. Now, he's just the guy we root against, but at the same time, I think most here appreciate just how bloody good he is. How can you not.
