Hello, Last night our two cats have become aggressive over each other. They have been living together fine for the past 6+ years until suddenly last night. One of the cats (cat 1) were sleeping on the bed while the other cat (cat 2) walked in. This is normally ok if cat 2 does not get on the bed. All of a sudden they started hissing and fighting each other. This went on all through the night. Now cat 2 is hiding in one of the bedrooms, while cat 1 is outside the door making sure she does not come out. Cat 2 that is inside the room will constantly hiss whenever she sees cat 1, but cat 1 does not seem to retaliate in any way. These cats have been living fine together for years and this sudden change has surprised us. Cat 1 is an incredibly affectionate cat towards humans, constantly wanting attention, but does not seem to care when giving affection to cat 2. What could be the cause for this sudden aggression and dominance.