Why do my rats hate each other?

I have 2 male rats that I have to keep in seperate cages. One rat terrorizes the other rat when theyre together, chases him around the cage, and injures him. I had to seperate them when one rat had a huge gash on his back, given to him by the other rat. Why do they hate each other so much, and what can I do to get them to like each other, so I can keep them together?