Bears love salmon brains because that is the part of the fish that has the highest fat content. When fish are plentiful, and the bears are "feeling rich", they may eat brains from the fish they catch almost exclusively. This works out real good for the other forest inhabitants (e.g. birds, mustellids, bugs) because they get to clean up the leftovers. Even the plants eventually get a piece of the action when various critters spread the wealth via fecal deposits, or when the carcasses simply decay and fertilize the forest floor directly. If you want to learn more about salmon's role in forest ecolgy check this out.