John, I respectfully disagree
Speaking for myself, I am a greatly revised me.
I am the same, but different. I have allowed part of me to surface that I would never permit to surface before because I didn't want to draw attention to myself (my sense of humor, for example). I am more energized, and therefore require/expect more out of life. I am more adventurous. There is more.....
Combine that with my DH, who is the same as he has always been. He has not undergone a life transformation. He has had to keep up with all of my discoveries about myself. He tells me I am his NEW wife that he has not known before, combined with the same soul.
I think this is a MAJOR factor in a marriage.
I wouldn't say that my DH is secure or insecure. I think he is paying attention, supporting the changes, and going on this ride, unafraid. He is able to do that. Not everyone can.
Marriage is a far more complex relationship than any other (and I include life partners). It is an intense one, a constant one, a personal one.
My marriage is not perfect (I know of no perfect marriage), but it is, in fact, better than ever, and it gets better and better daily.