You two were involved in a past life. Whether it was as lovers or in some other capacity, your ideals in life didn't match up, and you had a hard time feeling united, as if you were on the same team, working toward the same goals. This could have been in a spiritual sense -- perhaps you were of two different religions and you had a hard time reconciling your beliefs -- or in a more physical or monetary sense -- maybe one of you wanted money, power and fame, and the other couldn't care less about those things.
In this lifetime, you've come together with that same dynamic still in place. You still have a hard time matching up when it comes to your beliefs, your ideals and your visions of the future; so why have you come back for another round? It might be that you still have something to teach to each other about the benefits of your own way of doing things. This is especially true if it's on a spiritual level; you could have some difficult but important lessons to learn, because you weren't able to learn them in the last lifetime.