In some past life or other, you two were involved -- but not in a nurturing or kind way. Those old feelings of mistrust carry over into this life, and you, especially, have to work hard not to treat this person with wariness. When you first met, you might have felt like it was possible to develop a close bond, but as time goes on, you don't feel much closer to each other than you were in the beginning.
You might even feel farther apart if things happen to wear away at your trusting bond. Other aspects between you that promote intimacy, trust and harmony help to bridge the gap that is left by this one, but if there are too many challenging aspects between you, this one added to the mix just might be the kicker. If this relationship is worth something to you both, you each have to put time and effort into it. If you were lovers in a past life who betrayed each other in some way, for example, you have to take extra care in this life not to give your lover any reason to feel insecure or jealous in the relationship. If you were rivals in a past life, in this one you have to make sure to really listen to them, to hear them out and not respond with anger and aggression when things don't go the way you'd wish.