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 find, as time goes on, that you have different goals in life. This isn't a problem in itself except that it could actually hinder you from really getting somewhere in your own life, especially if you hold yourself back in order to make room for your partner. You're not likely to do that unless you've developed a really strong connection with each other, though, which makes this all the harder to deal with, because by the time it becomes an issue, you're already deeply involved with each other.
Over time, it's 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 results is subtle, but you 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. Especially if this is a long-term bond, finding a way to compromise and work around each other is essential to the success of your relationship. Finding alternate routes toward your goals is actually very enlightening! You might end up going places you never would've thought of on your own.