What can we say about the dentist? Fish don't like him. People don't really seem to like him either. Seriously, we're surprised this guy has any patients at all.
We first meet the dentist while he's scuba diving on the Great Barrier Reef. He sees Nemo out there in the open water and bags him right up. And how does the dentist see this fish-napping?
DENTIST: Heh heh heh! Beauty, isn't he? I found that guy struggling for life out on the reef and I saved him.
Um, okay. So the dentist clearly cannot tell the difference between a dying fish and one who's attempting to swim back to his home and family. That's not a point in his favor. Not only did he steal Nemo away from his dad, he's planning on passing along the struggling little fish to his fish-killing niece, Darla. Huh?
Overall, the dentist seems like a pretty clueless guy. He's mostly into himself and the things he likes—such as collecting exotic fish for his own amusement. He's really not that great of a dentist either. His patients are constantly moaning in agony or getting teeth yanked out forcefully. Sure, going to the dentist is never fun, but it's definitely a pain when you've got this guy on the other side of the drill.
Fun side note: The Pixar Wiki says that the dentist's real name is Philip Sherman. That makes sense since the diving mask that drops off his boat has the address for a "P. Sherman" written on them.
Hey, that's one thing we can say for the dentist. His labeling methods did turn out to be pretty handy for Marlin.