Obviously, most of the people in graduate school were already pretty interested with math to begin with. What I've heard, though, is that people always get interested in a subject if they go deep enough into it. You'll become interested in something if you get a deep understanding of it, even if you were bored of that thing in the first place. Graduate school, provided you actually try hard and learn the material, forces you to get a deep understanding of the math you had already learned through undergraduate school. Seems to me like that deep understanding would be guaranteed to give you some kind of passion or interest about math. Can any graduate students attest to this?