Sense of identity

Stand together yet not too near together:
For the pillars of the temple stand apart...

Kahlil Gibran

It's quite impossible to tell another person all of who you are.

It's also quite possible to be completely open about who you are without losing your sense of self (in fact, being completely open requires that you have a 'self')

Secrets on the other hand, are a different kettle of fish. We shouldn't share other peoples secrets shared with us in confidence.

And, because we are human, and make mistakes, and grow past those mistakes...there is no reason to share our secret mistakes with another, even a husband/wife. When they meet us - they fall in love with us for who we are now, not who we were in the past.

And there are some secrets that simply are no business of anyone else, husband/wife included.

It's actually incredibly common for couples to keep secrets from each other. Usually it's only younger people who don't realise the need - perhaps because they have been filled with an idealic sense of romance, rather than a real sense of who we are as people. Choosing which secrets to keep is about a sense of self (ie you know who you are and what you'd like to share or not), and a sense of judgement (you know who the other person is, and whether or not you 'should' share it with them)

I agree with Gibran on that, though I got sick of hearing about him in his popular heyday.

Also it is essentially boring to know every possible little thing about someone, even a mate.

― G.I. Gurdjieff

....and what you might observe in "yourself" will also be true of your partner.