Just like you, your partner is moving along on their very own, highly personal path of soul-growth. They need certain things for their evolution through the course of the current life, but if you become involved, you'll both find that the kind of love you're able to give them isn't the kind they need to propel them forward toward their future. This doesn't mean that a relationship between you won't be compelling, or feel downright wonderful; you may both feel as if this is the love you've been seeking.
But your partner will be moving away from their rightful path by becoming intimately involved with you. It's no one's fault; it doesn't even necessarily have roots in a mutual past-life love affair. It's more that your paths of personal evolution are different, and the style of love -- of partnership, of affection, of relationship values -- that comes naturally to you will somehow inhibit your lover's growth at a deeply spiritual level. If you truly care for them, you'll help them to stay true to their own needs and path, even as you continue to love them in the ways that you know how.