Though you might think for a while that you have a lot in common in terms of where you've been and where you're going, you'll find, as you get to know each other better, that you have different goals in life. Over time, it will be more and more difficult to find commonality between you, or any way for both of you to pursue your own goals at the same time without stepping on each other's toes or otherwise hurting each other. The tension that will result will be subtle, but you'll both feel it.
There is a silver lining to this cloud; the differences between you could actually force you both to become much more creative and cooperative with each other about reaching your own goals. Finding alternate routes toward your goals will actually be very enlightening! You might end up going places you never would've thought of on your own.