You each have goals and ambitions that you've developed over time, through careful work and consideration. Thus, it is all the more difficult to deal with the fact that, in a relationship with each other, you're likely to find that you work at cross-purposes to each other's ambitions. You want to support each other's careers, but you just aren't able to.
Time could be an issue -- you both are so busy doing your own thing that you just aren't able to be the companion or audience that your lover is looking for -- or it could be more about morality or ethics. A cop would have a hard time supporting the career of their con-artist lover, right? This is just the kind of issue you two run into. Your drive for success could run directly against your lover's wish to start a home and a family, or vice versa. Also, any cultural or familial differences between you show up as potentially glaring issues. Your pasts are quite different, and you lack the sense of commonality that could make you feel like allies, as different as you might be. Making this work long-term requires a lot of patience on both sides.