Since carrots aren't a rabbit's normal food and are thus likely to cause health problems if overconsumed, and [given] Bugs' [unhealthy habit of] eating them, his "What's up, Doc?" is his way of acknowledging that eating carrots will lead to hospitalization (the Doc), but that he doesn't care because they're so good. Its similar to a [heavy drinker] saying that their next beer is going straight to the liver. They know its bad, and likely to cause liver problems, but the alcohol is so good and makes things better, even if for a little bit, that the likelihood of [passing] is worth it.