You feel so comfortable together that you don't immediately notice the challenges this aspect brings to your relationship. You might feel as if you've known each other forever, even when you first met; as if you've finally found the person you've been looking for all along. In many ways, this is true.
You never stop finding new things that you have in common. Your goals and dreams for a relationship are in sync; you have the same desires in terms of all the biggies -- sexuality, children, living situation, spirituality and so on. The smaller things are in tandem, too. This all sounds wonderful, and it is. The problem lies in the fact that such commonality might encourage you to get it all laid out in the beginning -- we want this many kids at this point in the relationship; at this age we'll buy a house together, at this age we'll sell it and upgrade... You might end up with everything so pat and perfect that there's no room to grow. What happens when one of you has an urge that leads you in a direction away from the relationship? You might end up feeling as if don't know your partner at all -- that this person you thought you knew so well is actually an unknown entity. Which is actually more as it should be. Try not to limit each other by planning everything too closely for comfort.