Very seldom with the Black Bear. They deserve respect, but are nothing like a Grizzly. While they can become aggressive, they typically are timid scavengers.
I have lots of experience with these types of bear, having spent a good portion of my youth backpacking throughout the Sierras. I've had bear encounters, including a time when a black bear walked into my camp in the middle of the day and walked off with my fishing bait. In that case, I stupidly left my supplies unattended for two minutes and chased him off by banging on a pan.
I've seen these type of bear break car windows in parked vehicles in order to acquire food. While I know over the years there have been a couple of reports of a bad confrontation, these bear are easily dealt with by just being smart, and using the food storage devices located throughout the Sierras.
Yes some of the bears get conditioned to associate people with food. Its nearly always the fault of stupid people.
The black bears are becoming more common here in suburban
King Co WA. A cub was hit by a car on the freeway
near Bellevue, and a grizzly was photgraphed
off the North Cascades Hwy a couple of weeks ago.
Kind of neat that we have bears nearby, but they are
big enough and hungry enough to be a pain if one is
careless with food and garbage.
More people are killed by black bears than by grizzlys, probably because there are so many more black bears than grizzlys.
The problem is that many large predators, over protected in many areas, have not learned to fear man. Many have learned to associate man with food. Hence, the increasing number of bear, lion, and even rarer, wolf, attacks.