Specific bears are more likely to attack than others. In general, bears are only dangerous if a person does not obey the warning signs a bear gives them, or if the person makes sudden movements and high-pitched noises which could cause the bear to think they are prey. Black bears are more dangerous than brown bears.
Grizzly bears from Yellow Stone Park are in no way afraid of humans because they are accustomed to having them around and if anything, these grizzly bears are just curious. The meanest of all bears is said to be the Malaysian Sun Bear, which is quite aggressive and they will often go out of their way to attack humans.