You two are such a mismatch when it comes to what's important to you in life that you probably had a difficult time coming together as a couple in the first place. You're just on two completely different pages; you're not even opposed, like an atheist and a priest or a war protester and an Army general; you're more like ... a kangaroo and a generator.
A mathematician and a fisherman. You're just apples and oranges, and you're almost completely unable to understand the other's passion for their ideals and vocation. There's not much to say about this kind of mismatch; without some other really good aspects, this one is just plain hard. Even after just one email, one phone call, one date, you two realized this would be a struggle -- you're too different! And there are certainly plenty of times in this relationship where you both feel you're beating a dead horse, trying to make something happen that just wasn't meant to be; but, then again, sometimes a challenge is just what you need to add a little spice to your life.