I think the rivalry is caused by respect. Warlocks and Hunters differ but are also very similar and even complement each other. Warlocks are concerned with understanding the world around them. Hunters seem more concerned with exploring the world around them. How could you understand something without exploring it? How could you explore something without understanding it? In a way, their rivalry will always cause them to try to "One-up" the other while making sure to encourage the other to get better. I find it very amusing that both classes do the same motion whenever they use their Solar Supers.