You have a lot to teach this person; in fact, this is a really important relationship for them, and its effects last even if the affair doesn't. They are very comfortable with you; they could even feel as if they've known you for years -- it's even possible that you did know each other, long ago, or that you've shared friends in common. This sense of comfort and familiarity could also simply come from a commonality between you, a feeling of natural unity and warmth.
This person feels with you as if they're able to be themselves and follow their own path, even while respecting the loving bond between you -- and that's rare, so you both enjoy it! Talking about your experiences and allowing your lover to draw from your past is very beneficial for them, and it should be pleasurable for you. They make you feel wonderful because they're so grateful to you for inspiring them on a daily basis, which you do. You show them how to live better, to be the person they really are instead of always trying to please everybody else. And the way that they are naturally pleases you, so they're able to relax in your company and settle into themselves.