In a subtle way and one that isn't your lover's fault at all, they end up being an obstacle to your life path. They don't intentionally divert you from your true calling; in fact, they really want to support you in everything you do, especially as you two get closer as a couple. But they might focus too much on your past, which impedes your progress into the future.
They really identify with your past; maybe you've had similar upbringings, or you've had your hearts broken in ways that affected you similarly. You talk about these experiences and you find that they've shaped your values and the people that you've become. Whereas your lover wants security out of this relationship as a result of their past, that might not be your primary aim. Even if it is, providing that security for them might mean you can't move forward in your life. What if your dream is to join the Peace Corps and move to Africa? Or, less dramatic but equally valid, what if the right thing for you to be doing right now is simply dating around and not tying yourself down to one person? Again, this person doesn't hold you back on purpose, but the effects are real just the same. Your bond is a comfort to both of you, so it's up to you to recognize this stagnation when it happens, and push yourself to move forward in your own lives -- even if it compromises the security of your relationship.