Your carefully developed goals and ambitions could make it really difficult to accept the fact that you're likely to find that your partner's goals and ambitions are somehow incompatible with yours. Even if you want to support each other's careers, you just aren't able to, and this could be a divisive enough issue that it makes the whole relationship not worth pursuing for either of you. You have very different ideas of what you want to do in your life and what accomplishments make you feel like a success; for one of you, raising a family is the ultimate accomplishment, while the other doesn't have time for a spouse and kids because they're too busy building their career.
Or, you might just pursue two very different areas of success that have nothing to do with each other and no intersection. Public success for one of you inevitably means that the other is left out, which could leave that person feeling hurt and neglected. Making this work long-term requires a lot of patience on both sides, but you can do it if it's important to you. It means compromising your goals for the good of the relationship, and you each have to weigh whether that's worth it to you. Supporting each other's success, even if it means a temporary delay in finding your own, is ultimately the best choice to make if it's a relationship that's important to you.