The sense that you've known each other for a lot longer than you really have encourages your lover to idealize you, to feel as if this relationship were fated to be. They look at you through rose-colored glasses, at least in the beginning, convincing themselves that this is The One -- that this relationship is the most beautiful and meaningful of their life. From your first meeting, they rave to their friends -- We have so much in common!
This is so amazing! But part of this feeling comes from the fact that you two were actually involved long ago -- not in this life, but in a past one. And your relationship was similarly afflicted back then. Then, as now, your partner took it much more seriously than it deserved, and their heart was broken by the web of lies and deceit that you spun around them. In this lifetime, you may not deliberately set out to deceive this person, but you do so anyway, just by portraying an image of yourself that's not entirely correct. Yes, a part of each of you is beautiful, noble, loving and generous, but you're still human -- you have your selfish moments, your gloomy moods, your less than noble impulses. To be fully understood as a human being is a wonderful feeling, but it requires a lover who can handle the fact that you're not perfect. Are you two able to see each other that holistically? It's doubtful.