You tried to rely on this person in another lifetime, and they let you down in a big way. Whether they were your lover, your parent or your fellow warrior, when you needed them most, they weren't there for you, and that experience left a scar on your soul. In this life, your subconscious mind remembers that experience, and it will do anything to prevent the same thing from happening again.
Thus, on the conscious level, you'll look at your lover and see the picture of irresponsibility. You'll think, 'I have so much to teach them' -- after all, you take care of the things you need to take care of in your life, and in relationships, you do what you need to do to uphold your end of things. And so you'll go about trying to teach them all your wisdom regarding being a reliable partner and lover. Except your partner won't want to submit to your lessons; they're on their own path in life, and they may not agree with all the things you think are essential to feeling safe and secure in life as well as in love. The more they resist your lessons, the harder you'll push, but paradoxically, the harder you push them, the further they'll run away from you -- creating the exact situation you were trying to avoid in the first place. This connection offers you the chance to resolve that old, painful karma once and for all, though. You'll just have to learn to keep your partner by your side with sweetness and genuine attachment rather than through a sense of obligation or obedience to a set of 'relationship rules' that you set up for them to follow.