It's certainly important to know your subject - but that can lead to being "too comfortable".
Wild animals - and domestic animals - can do things that are unexpected. Many of these animals are incredibly powerful and very fast for short distances. Alligators can move at speeds of 35 mph on land for short distances, and 20 mph for longer distances in water. That's faster than a dolphin. A moose can run as fast as 35 mph and weighs 1500 pounds. Bears can reach speeds of 30 mph for short distances - and arrive with teeth and claws. In the Smokies, the even the bears know to avoid the elk in calving season (I'm not sure why tourists get too close, but that's another question). Snakes can move at speeds of 15 mph and faster, but venomous snake strikes occur in 1/24 second - about the speed of a blink of your eye and too fast to evade.
My concern when I'm in the field is snakes. I've been around snakes for a long time and don't fear them, but being surprised by a snake is different. When a 5 foot watersnake is coming directly at you, my response is to turn and run - not stop to verify that its a harmless watersnake and not a venomous water moccasin.
And I have walked in western parks and seen rattlesnakes quietly resting a couple of feet from a busy sidewalk.