 For a variety of reasons, he was starting to fall out of favor with the public, approval numbers being what they are. They were dropping, and Caesar calculated, probably correctly, hey, what I need is a war. What the Romans need are victories. They need news of Caesar's legions conquering Parthia. He was preparing for a war because of, if you're to believe the writing, which is all we can do, because of his sagging approval numbers.