So I went to get a second opinion yesterday. Apparently this doctor also thinks it'll go away on its own. But not for good; it should come back at some point. And he said that only about 10% of people he sees with this problem have a cause, the rest just have it for seemingly no reason.
He looked at my MRI and said that if there was a tumor or blood vessel, he'd be able to see it slightly distorting the brain stem. And if he saw anything like that, he'd get me an MRA test. But he thought that it looked fine.
So I guess I'll stick with the first neurologist. Even though I like the second one a bit better because he explained things better, he also said that he wasn't taking cases like mine as much and that the first doctor I saw was more of an expert.
I'm pretty disappointed that there's nothing I can do to cure it. In any other case, it would be really weird to say, but now I'm thinking "Crap, a wish it was a tumor."