Love is a Many Splendid Thing?

Just thinking about my next article and having some trouble.
How would you define the sublimation of the two different personalities into one unit in love?
I'm trying to pinpoint a secret I seem to see in the best relationships, which is not so much a common interest but a willingness to try the others interests or ideas, the fact that people in loving relationships are able to re-define themselves so easily while in those relationships.
Sort of contra-wise to those relationships were each party insists on independence and feel the need to define themselves as separate.
To my mind, the first is a much healthier relationship. What do you think?