Why can't bears and tigers get along with each other?

Tiger does not eat bear as food, and neither does bear eat a tiger as food but in areas where they live within the same geographic area like india and siberia, they always fight. The fights are of very violent and brutal nature, and often results in the death of 1 from the two. Recently a full grown black bear killed 3 tigers for unknown reason. Footage of the fight was shown on animal channel too. My question is that if they're not fighting for food with each other, then what are they fighting about? Also shouldn't they be used to each other since for centuries they have been living side by side?