How is a relationship built on a sound foundation?

My first thought was my oldest girl and her fella. Any unpleasant thought or feeling, big or small, gets brought up, acknowledged, and discussed and they somehow manage to do this sensibly. They are each self-aware and honest about their own flaws and issues and the roles those play in any difficulties, and I think a foundation like that can withstand a great deal. Even the effort to achieve that kind of communication could carry a pair far I think as it seems to me the desire might be more important than a perfect execution.