Actually the reason might be your fault. Cats are territorial, you can't just bring one home and think the original cat is just going to give up land and power. The way to introduce another pet is to keep the new pet in a room for about a month where it will eat, sleep and stay while the cats get to know each other by smell and sound. Curiosity will take over and the original cat will want to meet the new one. Then they will establish who is the boss and once that is done they can live happy ever after.