I'd say read Stephen Herrera's book on bear behavior. For a while you become a little paranoid, but, one of the things he differentiates is the difference between grizzly and black behavior - you never climb a tree with a black bear because they can climb very, very well. Grizzlies are generally too big and heavy and have claws that are not suited to the task.

In California, the only thing I worry about is injuring myself or becoming ill, hypothermic, or dehydrated. It is not reasonable to be afraid of black bears - respect them and keep your distance, and store your food properly. In the national parks, never leave your pack by itself - because your food is what the bears are really after. The times black bears here have injured people have generally been when they ignorantly leave food in their tent - RJ Secor notes in his book about the High Sierra that a hiker had to exit abruptly when a bear in Vidette Meadows (along the John Muir Trail) bit off his ear. Seems he used his food bag as a pillow instead of using one of the bear lockers to store it.

In Alaska, I would never go anywhere without other people - preferably well armed people - and bear spray. The black bears have been known to predate on people there. And the grizzlies have been known to as well.

In short - information is your friend. Bears have regional and species differences. I don't think you need to be so fearful if you take reasonable precautions based on information provided by the local jurisdiction - you won't need bear spray in Yosemite, but you will probably be encouraged to carry it in Wyoming, Montana or Alaska. Store your food as directed - in grizzly areas, cables are often installed for hanging food. Here in California the parks publish lists of bear canisters they approve.