Patience.
Too many people do not have patience these days.
The patience to work things through, the patience to listen to their spouse or partner, the patience to calmly and rationally discuss the issues.
People are quick to anger today, usually become defensive right away, feel as if they are being attacked and of course will lash back out themselves.
While communication is key as others have said, I believe you must have patience first before you can get to the point of communicating. There is no point trying to communicate when you, your partner or both of you are feeling angry or resentful. The communication will simply devolve to arguing and name calling.
You must have patience and understanding of the others view point. Notice I said understanding not agreeing. It is always OK to disagree with someone as long as it is done politely and respectfully, in a way that does not make them feel belittled or ignored.
Patience is the key.