Those who spend a lot of time thinking big thoughts about what makes successful relationships work like to talk about enmeshment vs. disengagement.
On the hot side are the enmeshed couples. They tend to have blurred personal boundaries. They are emotionally dependent on one another. They need the other person to complete them and make them feel good about themselves. They need their spouses' continual support and assurance, and will accept mistreatment or easily give in to make peace, will give up what's important to them under pressure. Most of all, if one of them gets anxious or upset, it's contagious - their partner gets rattled as well.
On the cold side are those who have a rugged self-reliance. The relationship is disengaged. They rely on no one, not even each other. I take care of me, you take care of you. They are not open, trusting, or truly intimate. As a couple they often live parallel lives - coming together, say, around kids - but rarely connect.
The middle ground between the too hot and the too cold is what is known as self-differentiation. Here are some of the qualities of these folks:
• Are able to be emotionally calm even when the other person is upset.
• Are able to step back and see problems in the relationship and make positive changes without any expectations of the other.
• See others as anxious or fearful rather than malicious or manipulative in conflict
• Are able to not react in kind to anger or another's anxiety
• Are able to hold to their values and choices even if others disagree or are disapproving
• Are able to focus on their personal responsibility rather than the behavior of the other in the relationship.
You can almost feel the difference here. They stand solidly on their own two feet. They focus on changing themselves, rather than trying to get others to change. They aren't rattled when those around them are having a hard time, but can be empathic and helpful.
To get to this balanced middle-place requires adapting a certain mindset and practice. The mindset is one of focusing more on what you can control and do, rather than what the other guy should do; about being clear with others about how you feel and holding to what is important to you; seeing other's problems as their problems - you can help, but don't need to get threatened yourself or feel compelled to fix their problems for them.
Practice is...well practice. Try standing in this middle ground, and see what happens.