If you think back through your past and present relationships, consider those things which have robbed you of intimacy. To know our own personal dialogue is one of our biggest challenges. If we are able to, I think we can understand this quote quite well.
It is no surprise that judging the other in a relationship and rejecting them, as well as being judged and rejected ourselves, would spoil intimacy. The surprise comes in Mason's answer - love.
I think that this is marriage's greatest gift to us - the gift of learning how to love. Yes, it is trial and much error. Yes, it is learning when we feel rejected and condemned and reaching back in love. Yes, it is in recognizing when we act out of insecurity and defensiveness, judging, condemning, rejecting our partner - and choosing to love instead.
This is a daily thing. It is the small hurts that add up and steal our intimacy. And it is the small, moment by moment, acts of love, turning to the other, turning away from bitterness, that can bring it back again.