In a past life, you held dominance over this person: You controlled them, bullied them or even battled them in war. Whatever your relationship may have been then -- master and student, parent and child, siblings, lovers, enemies -- it was filled with animosity and characterized by power struggles. Thus, even if you get along in this life like best friends, that old, tricky dynamic will still be felt from time to time.
Perhaps without realizing it, you'll slip back into that dominant role once again, either by trying to control how your lover spends their time, how they dress, what they eat and the like, or by pushing at them to open up emotionally in ways that make them uncomfortable. You'll try to gain the upper hand in the relationship, just as you did in a past life, and you're not likely to be sensitive to your partner's feelings in the matter...that is, unless you can remind yourself that this is a dynamic you don't want to repeat. The way to resolve this particular type of karmic baggage is not to poke at it like a bruise, but simply to correct your behavior. Adopt a position of compassion instead of control, and listen to and respect your partner's feelings -- and their boundaries.